x86/entry/64: Fix paranoid_entry() frame pointer warning

[sfrench/cifs-2.6.git] / arch / x86 / entry / entry_64.S

/* SPDX-License-Identifier: GPL-2.0 */
/*
 *  linux/arch/x86_64/entry.S
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *  Copyright (C) 2000, 2001, 2002  Andi Kleen SuSE Labs
 *  Copyright (C) 2000  Pavel Machek <pavel@suse.cz>
 *
 * entry.S contains the system-call and fault low-level handling routines.
 *
 * Some of this is documented in Documentation/x86/entry_64.txt
 *
 * A note on terminology:
 * - iret frame:        Architecture defined interrupt frame from SS to RIP
 *                      at the top of the kernel process stack.
 *
 * Some macro usage:
 * - ENTRY/END:         Define functions in the symbol table.
 * - TRACE_IRQ_*:       Trace hardirq state for lock debugging.
 * - idtentry:          Define exception entry points.
 */
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/cache.h>
#include <asm/errno.h>
#include <asm/asm-offsets.h>
#include <asm/msr.h>
#include <asm/unistd.h>
#include <asm/thread_info.h>
#include <asm/hw_irq.h>
#include <asm/page_types.h>
#include <asm/irqflags.h>
#include <asm/paravirt.h>
#include <asm/percpu.h>
#include <asm/asm.h>
#include <asm/smap.h>
#include <asm/pgtable_types.h>
#include <asm/export.h>
#include <asm/frame.h>
#include <asm/nospec-branch.h>
#include <linux/err.h>

#include "calling.h"

.code64
.section .entry.text, "ax"

#ifdef CONFIG_PARAVIRT
ENTRY(native_usergs_sysret64)
        UNWIND_HINT_EMPTY
        swapgs
        sysretq
END(native_usergs_sysret64)
#endif /* CONFIG_PARAVIRT */

.macro TRACE_IRQS_FLAGS flags:req
#ifdef CONFIG_TRACE_IRQFLAGS
        bt      $9, \flags              /* interrupts off? */
        jnc     1f
        TRACE_IRQS_ON
1:
#endif
.endm

.macro TRACE_IRQS_IRETQ
        TRACE_IRQS_FLAGS EFLAGS(%rsp)
.endm

/*
 * When dynamic function tracer is enabled it will add a breakpoint
 * to all locations that it is about to modify, sync CPUs, update
 * all the code, sync CPUs, then remove the breakpoints. In this time
 * if lockdep is enabled, it might jump back into the debug handler
 * outside the updating of the IST protection. (TRACE_IRQS_ON/OFF).
 *
 * We need to change the IDT table before calling TRACE_IRQS_ON/OFF to
 * make sure the stack pointer does not get reset back to the top
 * of the debug stack, and instead just reuses the current stack.
 */
#if defined(CONFIG_DYNAMIC_FTRACE) && defined(CONFIG_TRACE_IRQFLAGS)

.macro TRACE_IRQS_OFF_DEBUG
        call    debug_stack_set_zero
        TRACE_IRQS_OFF
        call    debug_stack_reset
.endm

.macro TRACE_IRQS_ON_DEBUG
        call    debug_stack_set_zero
        TRACE_IRQS_ON
        call    debug_stack_reset
.endm

.macro TRACE_IRQS_IRETQ_DEBUG
        bt      $9, EFLAGS(%rsp)                /* interrupts off? */
        jnc     1f
        TRACE_IRQS_ON_DEBUG
1:
.endm

#else
# define TRACE_IRQS_OFF_DEBUG                   TRACE_IRQS_OFF
# define TRACE_IRQS_ON_DEBUG                    TRACE_IRQS_ON
# define TRACE_IRQS_IRETQ_DEBUG                 TRACE_IRQS_IRETQ
#endif

/*
 * 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
 *
 * This is the only entry point used for 64-bit system calls.  The
 * hardware interface is reasonably well designed and the register to
 * argument mapping Linux uses fits well with the registers that are
 * available when SYSCALL is used.
 *
 * SYSCALL instructions can be found inlined in libc implementations as
 * well as some other programs and libraries.  There are also a handful
 * of SYSCALL instructions in the vDSO used, for example, as a
 * clock_gettimeofday fallback.
 *
 * 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
 * then loads new ss, cs, and rip from previously programmed MSRs.
 * rflags gets masked by a value from another MSR (so CLD and CLAC
 * are not needed). SYSCALL does not save anything on the stack
 * and does not change rsp.
 *
 * Registers on entry:
 * rax  system call number
 * rcx  return address
 * r11  saved rflags (note: r11 is callee-clobbered register in C ABI)
 * rdi  arg0
 * rsi  arg1
 * rdx  arg2
 * r10  arg3 (needs to be moved to rcx to conform to C ABI)
 * r8   arg4
 * r9   arg5
 * (note: r12-r15, rbp, rbx are callee-preserved in C ABI)
 *
 * Only called from user space.
 *
 * When user can change pt_regs->foo always force IRET. That is because
 * it deals with uncanonical addresses better. SYSRET has trouble
 * with them due to bugs in both AMD and Intel CPUs.
 */

        .pushsection .entry_trampoline, "ax"

/*
 * The code in here gets remapped into cpu_entry_area's trampoline.  This means
 * that the assembler and linker have the wrong idea as to where this code
 * lives (and, in fact, it's mapped more than once, so it's not even at a
 * fixed address).  So we can't reference any symbols outside the entry
 * trampoline and expect it to work.
 *
 * Instead, we carefully abuse %rip-relative addressing.
 * _entry_trampoline(%rip) refers to the start of the remapped) entry
 * trampoline.  We can thus find cpu_entry_area with this macro:
 */

#define CPU_ENTRY_AREA \
        _entry_trampoline - CPU_ENTRY_AREA_entry_trampoline(%rip)

/* The top word of the SYSENTER stack is hot and is usable as scratch space. */
#define RSP_SCRATCH     CPU_ENTRY_AREA_entry_stack + \
                        SIZEOF_entry_stack - 8 + CPU_ENTRY_AREA

ENTRY(entry_SYSCALL_64_trampoline)
        UNWIND_HINT_EMPTY
        swapgs

        /* Stash the user RSP. */
        movq    %rsp, RSP_SCRATCH

        /* Note: using %rsp as a scratch reg. */
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp

        /* Load the top of the task stack into RSP */
        movq    CPU_ENTRY_AREA_tss + TSS_sp1 + CPU_ENTRY_AREA, %rsp

        /* Start building the simulated IRET frame. */
        pushq   $__USER_DS                      /* pt_regs->ss */
        pushq   RSP_SCRATCH                     /* pt_regs->sp */
        pushq   %r11                            /* pt_regs->flags */
        pushq   $__USER_CS                      /* pt_regs->cs */
        pushq   %rcx                            /* pt_regs->ip */

        /*
         * x86 lacks a near absolute jump, and we can't jump to the real
         * entry text with a relative jump.  We could push the target
         * address and then use retq, but this destroys the pipeline on
         * many CPUs (wasting over 20 cycles on Sandy Bridge).  Instead,
         * spill RDI and restore it in a second-stage trampoline.
         */
        pushq   %rdi
        movq    $entry_SYSCALL_64_stage2, %rdi
        JMP_NOSPEC %rdi
END(entry_SYSCALL_64_trampoline)

        .popsection

ENTRY(entry_SYSCALL_64_stage2)
        UNWIND_HINT_EMPTY
        popq    %rdi
        jmp     entry_SYSCALL_64_after_hwframe
END(entry_SYSCALL_64_stage2)

ENTRY(entry_SYSCALL_64)
        UNWIND_HINT_EMPTY
        /*
         * Interrupts are off on entry.
         * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
         * it is too small to ever cause noticeable irq latency.
         */

        swapgs
        /*
         * This path is only taken when PAGE_TABLE_ISOLATION is disabled so it
         * is not required to switch CR3.
         */
        movq    %rsp, PER_CPU_VAR(rsp_scratch)
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp

        /* Construct struct pt_regs on stack */
        pushq   $__USER_DS                      /* pt_regs->ss */
        pushq   PER_CPU_VAR(rsp_scratch)        /* pt_regs->sp */
        pushq   %r11                            /* pt_regs->flags */
        pushq   $__USER_CS                      /* pt_regs->cs */
        pushq   %rcx                            /* pt_regs->ip */
GLOBAL(entry_SYSCALL_64_after_hwframe)
        pushq   %rax                            /* pt_regs->orig_ax */

        PUSH_AND_CLEAR_REGS rax=$-ENOSYS

        TRACE_IRQS_OFF

        /* IRQs are off. */
        movq    %rsp, %rdi
        call    do_syscall_64           /* returns with IRQs disabled */

        TRACE_IRQS_IRETQ                /* we're about to change IF */

        /*
         * Try to use SYSRET instead of IRET if we're returning to
         * a completely clean 64-bit userspace context.  If we're not,
         * go to the slow exit path.
         */
        movq    RCX(%rsp), %rcx
        movq    RIP(%rsp), %r11

        cmpq    %rcx, %r11      /* SYSRET requires RCX == RIP */
        jne     swapgs_restore_regs_and_return_to_usermode

        /*
         * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
         * in kernel space.  This essentially lets the user take over
         * the kernel, since userspace controls RSP.
         *
         * If width of "canonical tail" ever becomes variable, this will need
         * to be updated to remain correct on both old and new CPUs.
         *
         * Change top bits to match most significant bit (47th or 56th bit
         * depending on paging mode) in the address.
         */
        shl     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
        sar     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx

        /* If this changed %rcx, it was not canonical */
        cmpq    %rcx, %r11
        jne     swapgs_restore_regs_and_return_to_usermode

        cmpq    $__USER_CS, CS(%rsp)            /* CS must match SYSRET */
        jne     swapgs_restore_regs_and_return_to_usermode

        movq    R11(%rsp), %r11
        cmpq    %r11, EFLAGS(%rsp)              /* R11 == RFLAGS */
        jne     swapgs_restore_regs_and_return_to_usermode

        /*
         * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
         * restore RF properly. If the slowpath sets it for whatever reason, we
         * need to restore it correctly.
         *
         * SYSRET can restore TF, but unlike IRET, restoring TF results in a
         * trap from userspace immediately after SYSRET.  This would cause an
         * infinite loop whenever #DB happens with register state that satisfies
         * the opportunistic SYSRET conditions.  For example, single-stepping
         * this user code:
         *
         *           movq       $stuck_here, %rcx
         *           pushfq
         *           popq %r11
         *   stuck_here:
         *
         * would never get past 'stuck_here'.
         */
        testq   $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
        jnz     swapgs_restore_regs_and_return_to_usermode

        /* nothing to check for RSP */

        cmpq    $__USER_DS, SS(%rsp)            /* SS must match SYSRET */
        jne     swapgs_restore_regs_and_return_to_usermode

        /*
         * We win! This label is here just for ease of understanding
         * perf profiles. Nothing jumps here.
         */
syscall_return_via_sysret:
        /* rcx and r11 are already restored (see code above) */
        UNWIND_HINT_EMPTY
        POP_REGS pop_rdi=0 skip_r11rcx=1

        /*
         * Now all regs are restored except RSP and RDI.
         * Save old stack pointer and switch to trampoline stack.
         */
        movq    %rsp, %rdi
        movq    PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp

        pushq   RSP-RDI(%rdi)   /* RSP */
        pushq   (%rdi)          /* RDI */

        /*
         * We are on the trampoline stack.  All regs except RDI are live.
         * We can do future final exit work right here.
         */
        SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

        popq    %rdi
        popq    %rsp
        USERGS_SYSRET64
END(entry_SYSCALL_64)

/*
 * %rdi: prev task
 * %rsi: next task
 */
ENTRY(__switch_to_asm)
        UNWIND_HINT_FUNC
        /*
         * Save callee-saved registers
         * This must match the order in inactive_task_frame
         */
        pushq   %rbp
        pushq   %rbx
        pushq   %r12
        pushq   %r13
        pushq   %r14
        pushq   %r15

        /* switch stack */
        movq    %rsp, TASK_threadsp(%rdi)
        movq    TASK_threadsp(%rsi), %rsp

#ifdef CONFIG_CC_STACKPROTECTOR
        movq    TASK_stack_canary(%rsi), %rbx
        movq    %rbx, PER_CPU_VAR(irq_stack_union)+stack_canary_offset
#endif

#ifdef CONFIG_RETPOLINE
        /*
         * When switching from a shallower to a deeper call stack
         * the RSB may either underflow or use entries populated
         * with userspace addresses. On CPUs where those concerns
         * exist, overwrite the RSB with entries which capture
         * speculative execution to prevent attack.
         */
        /* Clobbers %rbx */
        FILL_RETURN_BUFFER RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
#endif

        /* restore callee-saved registers */
        popq    %r15
        popq    %r14
        popq    %r13
        popq    %r12
        popq    %rbx
        popq    %rbp

        jmp     __switch_to
END(__switch_to_asm)

/*
 * A newly forked process directly context switches into this address.
 *
 * rax: prev task we switched from
 * rbx: kernel thread func (NULL for user thread)
 * r12: kernel thread arg
 */
ENTRY(ret_from_fork)
        UNWIND_HINT_EMPTY
        movq    %rax, %rdi
        call    schedule_tail                   /* rdi: 'prev' task parameter */

        testq   %rbx, %rbx                      /* from kernel_thread? */
        jnz     1f                              /* kernel threads are uncommon */

2:
        UNWIND_HINT_REGS
        movq    %rsp, %rdi
        call    syscall_return_slowpath /* returns with IRQs disabled */
        TRACE_IRQS_ON                   /* user mode is traced as IRQS on */
        jmp     swapgs_restore_regs_and_return_to_usermode

1:
        /* kernel thread */
        movq    %r12, %rdi
        CALL_NOSPEC %rbx
        /*
         * A kernel thread is allowed to return here after successfully
         * calling do_execve().  Exit to userspace to complete the execve()
         * syscall.
         */
        movq    $0, RAX(%rsp)
        jmp     2b
END(ret_from_fork)

/*
 * Build the entry stubs with some assembler magic.
 * We pack 1 stub into every 8-byte block.
 */
        .align 8
ENTRY(irq_entries_start)
    vector=FIRST_EXTERNAL_VECTOR
    .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR)
        UNWIND_HINT_IRET_REGS
        pushq   $(~vector+0x80)                 /* Note: always in signed byte range */
        jmp     common_interrupt
        .align  8
        vector=vector+1
    .endr
END(irq_entries_start)

.macro DEBUG_ENTRY_ASSERT_IRQS_OFF
#ifdef CONFIG_DEBUG_ENTRY
        pushq %rax
        SAVE_FLAGS(CLBR_RAX)
        testl $X86_EFLAGS_IF, %eax
        jz .Lokay_\@
        ud2
.Lokay_\@:
        popq %rax
#endif
.endm

/*
 * Enters the IRQ stack if we're not already using it.  NMI-safe.  Clobbers
 * flags and puts old RSP into old_rsp, and leaves all other GPRs alone.
 * Requires kernel GSBASE.
 *
 * The invariant is that, if irq_count != -1, then the IRQ stack is in use.
 */
.macro ENTER_IRQ_STACK regs=1 old_rsp
        DEBUG_ENTRY_ASSERT_IRQS_OFF
        movq    %rsp, \old_rsp

        .if \regs
        UNWIND_HINT_REGS base=\old_rsp
        .endif

        incl    PER_CPU_VAR(irq_count)
        jnz     .Lirq_stack_push_old_rsp_\@

        /*
         * Right now, if we just incremented irq_count to zero, we've
         * claimed the IRQ stack but we haven't switched to it yet.
         *
         * If anything is added that can interrupt us here without using IST,
         * it must be *extremely* careful to limit its stack usage.  This
         * could include kprobes and a hypothetical future IST-less #DB
         * handler.
         *
         * The OOPS unwinder relies on the word at the top of the IRQ
         * stack linking back to the previous RSP for the entire time we're
         * on the IRQ stack.  For this to work reliably, we need to write
         * it before we actually move ourselves to the IRQ stack.
         */

        movq    \old_rsp, PER_CPU_VAR(irq_stack_union + IRQ_STACK_SIZE - 8)
        movq    PER_CPU_VAR(irq_stack_ptr), %rsp

#ifdef CONFIG_DEBUG_ENTRY
        /*
         * If the first movq above becomes wrong due to IRQ stack layout
         * changes, the only way we'll notice is if we try to unwind right
         * here.  Assert that we set up the stack right to catch this type
         * of bug quickly.
         */
        cmpq    -8(%rsp), \old_rsp
        je      .Lirq_stack_okay\@
        ud2
        .Lirq_stack_okay\@:
#endif

.Lirq_stack_push_old_rsp_\@:
        pushq   \old_rsp

        .if \regs
        UNWIND_HINT_REGS indirect=1
        .endif
.endm

/*
 * Undoes ENTER_IRQ_STACK.
 */
.macro LEAVE_IRQ_STACK regs=1
        DEBUG_ENTRY_ASSERT_IRQS_OFF
        /* We need to be off the IRQ stack before decrementing irq_count. */
        popq    %rsp

        .if \regs
        UNWIND_HINT_REGS
        .endif

        /*
         * As in ENTER_IRQ_STACK, irq_count == 0, we are still claiming
         * the irq stack but we're not on it.
         */

        decl    PER_CPU_VAR(irq_count)
.endm

/*
 * Interrupt entry/exit.
 *
 * Interrupt entry points save only callee clobbered registers in fast path.
 *
 * Entry runs with interrupts off.
 */

/* 0(%rsp): ~(interrupt number) */
        .macro interrupt func
        cld

        testb   $3, CS-ORIG_RAX(%rsp)
        jz      1f
        SWAPGS
        call    switch_to_thread_stack
1:

        PUSH_AND_CLEAR_REGS
        ENCODE_FRAME_POINTER

        testb   $3, CS(%rsp)
        jz      1f

        /*
         * IRQ from user mode.
         *
         * We need to tell lockdep that IRQs are off.  We can't do this until
         * we fix gsbase, and we should do it before enter_from_user_mode
         * (which can take locks).  Since TRACE_IRQS_OFF idempotent,
         * the simplest way to handle it is to just call it twice if
         * we enter from user mode.  There's no reason to optimize this since
         * TRACE_IRQS_OFF is a no-op if lockdep is off.
         */
        TRACE_IRQS_OFF

        CALL_enter_from_user_mode

1:
        ENTER_IRQ_STACK old_rsp=%rdi
        /* We entered an interrupt context - irqs are off: */
        TRACE_IRQS_OFF

        call    \func   /* rdi points to pt_regs */
        .endm

        /*
         * The interrupt stubs push (~vector+0x80) onto the stack and
         * then jump to common_interrupt.
         */
        .p2align CONFIG_X86_L1_CACHE_SHIFT
common_interrupt:
        ASM_CLAC
        addq    $-0x80, (%rsp)                  /* Adjust vector to [-256, -1] range */
        interrupt do_IRQ
        /* 0(%rsp): old RSP */
ret_from_intr:
        DISABLE_INTERRUPTS(CLBR_ANY)
        TRACE_IRQS_OFF

        LEAVE_IRQ_STACK

        testb   $3, CS(%rsp)
        jz      retint_kernel

        /* Interrupt came from user space */
GLOBAL(retint_user)
        mov     %rsp,%rdi
        call    prepare_exit_to_usermode
        TRACE_IRQS_IRETQ

GLOBAL(swapgs_restore_regs_and_return_to_usermode)
#ifdef CONFIG_DEBUG_ENTRY
        /* Assert that pt_regs indicates user mode. */
        testb   $3, CS(%rsp)
        jnz     1f
        ud2
1:
#endif
        POP_REGS pop_rdi=0

        /*
         * The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
         * Save old stack pointer and switch to trampoline stack.
         */
        movq    %rsp, %rdi
        movq    PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp

        /* Copy the IRET frame to the trampoline stack. */
        pushq   6*8(%rdi)       /* SS */
        pushq   5*8(%rdi)       /* RSP */
        pushq   4*8(%rdi)       /* EFLAGS */
        pushq   3*8(%rdi)       /* CS */
        pushq   2*8(%rdi)       /* RIP */

        /* Push user RDI on the trampoline stack. */
        pushq   (%rdi)

        /*
         * We are on the trampoline stack.  All regs except RDI are live.
         * We can do future final exit work right here.
         */

        SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

        /* Restore RDI. */
        popq    %rdi
        SWAPGS
        INTERRUPT_RETURN


/* Returning to kernel space */
retint_kernel:
#ifdef CONFIG_PREEMPT
        /* Interrupts are off */
        /* Check if we need preemption */
        bt      $9, EFLAGS(%rsp)                /* were interrupts off? */
        jnc     1f
0:      cmpl    $0, PER_CPU_VAR(__preempt_count)
        jnz     1f
        call    preempt_schedule_irq
        jmp     0b
1:
#endif
        /*
         * The iretq could re-enable interrupts:
         */
        TRACE_IRQS_IRETQ

GLOBAL(restore_regs_and_return_to_kernel)
#ifdef CONFIG_DEBUG_ENTRY
        /* Assert that pt_regs indicates kernel mode. */
        testb   $3, CS(%rsp)
        jz      1f
        ud2
1:
#endif
        POP_REGS
        addq    $8, %rsp        /* skip regs->orig_ax */
        INTERRUPT_RETURN

ENTRY(native_iret)
        UNWIND_HINT_IRET_REGS
        /*
         * Are we returning to a stack segment from the LDT?  Note: in
         * 64-bit mode SS:RSP on the exception stack is always valid.
         */
#ifdef CONFIG_X86_ESPFIX64
        testb   $4, (SS-RIP)(%rsp)
        jnz     native_irq_return_ldt
#endif

.global native_irq_return_iret
native_irq_return_iret:
        /*
         * This may fault.  Non-paranoid faults on return to userspace are
         * handled by fixup_bad_iret.  These include #SS, #GP, and #NP.
         * Double-faults due to espfix64 are handled in do_double_fault.
         * Other faults here are fatal.
         */
        iretq

#ifdef CONFIG_X86_ESPFIX64
native_irq_return_ldt:
        /*
         * We are running with user GSBASE.  All GPRs contain their user
         * values.  We have a percpu ESPFIX stack that is eight slots
         * long (see ESPFIX_STACK_SIZE).  espfix_waddr points to the bottom
         * of the ESPFIX stack.
         *
         * We clobber RAX and RDI in this code.  We stash RDI on the
         * normal stack and RAX on the ESPFIX stack.
         *
         * The ESPFIX stack layout we set up looks like this:
         *
         * --- top of ESPFIX stack ---
         * SS
         * RSP
         * RFLAGS
         * CS
         * RIP  <-- RSP points here when we're done
         * RAX  <-- espfix_waddr points here
         * --- bottom of ESPFIX stack ---
         */

        pushq   %rdi                            /* Stash user RDI */
        SWAPGS                                  /* to kernel GS */
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi   /* to kernel CR3 */

        movq    PER_CPU_VAR(espfix_waddr), %rdi
        movq    %rax, (0*8)(%rdi)               /* user RAX */
        movq    (1*8)(%rsp), %rax               /* user RIP */
        movq    %rax, (1*8)(%rdi)
        movq    (2*8)(%rsp), %rax               /* user CS */
        movq    %rax, (2*8)(%rdi)
        movq    (3*8)(%rsp), %rax               /* user RFLAGS */
        movq    %rax, (3*8)(%rdi)
        movq    (5*8)(%rsp), %rax               /* user SS */
        movq    %rax, (5*8)(%rdi)
        movq    (4*8)(%rsp), %rax               /* user RSP */
        movq    %rax, (4*8)(%rdi)
        /* Now RAX == RSP. */

        andl    $0xffff0000, %eax               /* RAX = (RSP & 0xffff0000) */

        /*
         * espfix_stack[31:16] == 0.  The page tables are set up such that
         * (espfix_stack | (X & 0xffff0000)) points to a read-only alias of
         * espfix_waddr for any X.  That is, there are 65536 RO aliases of
         * the same page.  Set up RSP so that RSP[31:16] contains the
         * respective 16 bits of the /userspace/ RSP and RSP nonetheless
         * still points to an RO alias of the ESPFIX stack.
         */
        orq     PER_CPU_VAR(espfix_stack), %rax

        SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
        SWAPGS                                  /* to user GS */
        popq    %rdi                            /* Restore user RDI */

        movq    %rax, %rsp
        UNWIND_HINT_IRET_REGS offset=8

        /*
         * At this point, we cannot write to the stack any more, but we can
         * still read.
         */
        popq    %rax                            /* Restore user RAX */

        /*
         * RSP now points to an ordinary IRET frame, except that the page
         * is read-only and RSP[31:16] are preloaded with the userspace
         * values.  We can now IRET back to userspace.
         */
        jmp     native_irq_return_iret
#endif
END(common_interrupt)

/*
 * APIC interrupts.
 */
.macro apicinterrupt3 num sym do_sym
ENTRY(\sym)
        UNWIND_HINT_IRET_REGS
        ASM_CLAC
        pushq   $~(\num)
.Lcommon_\sym:
        interrupt \do_sym
        jmp     ret_from_intr
END(\sym)
.endm

/* Make sure APIC interrupt handlers end up in the irqentry section: */
#define PUSH_SECTION_IRQENTRY   .pushsection .irqentry.text, "ax"
#define POP_SECTION_IRQENTRY    .popsection

.macro apicinterrupt num sym do_sym
PUSH_SECTION_IRQENTRY
apicinterrupt3 \num \sym \do_sym
POP_SECTION_IRQENTRY
.endm

#ifdef CONFIG_SMP
apicinterrupt3 IRQ_MOVE_CLEANUP_VECTOR          irq_move_cleanup_interrupt      smp_irq_move_cleanup_interrupt
apicinterrupt3 REBOOT_VECTOR                    reboot_interrupt                smp_reboot_interrupt
#endif

#ifdef CONFIG_X86_UV
apicinterrupt3 UV_BAU_MESSAGE                   uv_bau_message_intr1            uv_bau_message_interrupt
#endif

apicinterrupt LOCAL_TIMER_VECTOR                apic_timer_interrupt            smp_apic_timer_interrupt
apicinterrupt X86_PLATFORM_IPI_VECTOR           x86_platform_ipi                smp_x86_platform_ipi

#ifdef CONFIG_HAVE_KVM
apicinterrupt3 POSTED_INTR_VECTOR               kvm_posted_intr_ipi             smp_kvm_posted_intr_ipi
apicinterrupt3 POSTED_INTR_WAKEUP_VECTOR        kvm_posted_intr_wakeup_ipi      smp_kvm_posted_intr_wakeup_ipi
apicinterrupt3 POSTED_INTR_NESTED_VECTOR        kvm_posted_intr_nested_ipi      smp_kvm_posted_intr_nested_ipi
#endif

#ifdef CONFIG_X86_MCE_THRESHOLD
apicinterrupt THRESHOLD_APIC_VECTOR             threshold_interrupt             smp_threshold_interrupt
#endif

#ifdef CONFIG_X86_MCE_AMD
apicinterrupt DEFERRED_ERROR_VECTOR             deferred_error_interrupt        smp_deferred_error_interrupt
#endif

#ifdef CONFIG_X86_THERMAL_VECTOR
apicinterrupt THERMAL_APIC_VECTOR               thermal_interrupt               smp_thermal_interrupt
#endif

#ifdef CONFIG_SMP
apicinterrupt CALL_FUNCTION_SINGLE_VECTOR       call_function_single_interrupt  smp_call_function_single_interrupt
apicinterrupt CALL_FUNCTION_VECTOR              call_function_interrupt         smp_call_function_interrupt
apicinterrupt RESCHEDULE_VECTOR                 reschedule_interrupt            smp_reschedule_interrupt
#endif

apicinterrupt ERROR_APIC_VECTOR                 error_interrupt                 smp_error_interrupt
apicinterrupt SPURIOUS_APIC_VECTOR              spurious_interrupt              smp_spurious_interrupt

#ifdef CONFIG_IRQ_WORK
apicinterrupt IRQ_WORK_VECTOR                   irq_work_interrupt              smp_irq_work_interrupt
#endif

/*
 * Exception entry points.
 */
#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss_rw) + (TSS_ist + ((x) - 1) * 8)

/*
 * Switch to the thread stack.  This is called with the IRET frame and
 * orig_ax on the stack.  (That is, RDI..R12 are not on the stack and
 * space has not been allocated for them.)
 */
ENTRY(switch_to_thread_stack)
        UNWIND_HINT_FUNC

        pushq   %rdi
        /* Need to switch before accessing the thread stack. */
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
        movq    %rsp, %rdi
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
        UNWIND_HINT sp_offset=16 sp_reg=ORC_REG_DI

        pushq   7*8(%rdi)               /* regs->ss */
        pushq   6*8(%rdi)               /* regs->rsp */
        pushq   5*8(%rdi)               /* regs->eflags */
        pushq   4*8(%rdi)               /* regs->cs */
        pushq   3*8(%rdi)               /* regs->ip */
        pushq   2*8(%rdi)               /* regs->orig_ax */
        pushq   8(%rdi)                 /* return address */
        UNWIND_HINT_FUNC

        movq    (%rdi), %rdi
        ret
END(switch_to_thread_stack)

.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
ENTRY(\sym)
        UNWIND_HINT_IRET_REGS offset=\has_error_code*8

        /* Sanity check */
        .if \shift_ist != -1 && \paranoid == 0
        .error "using shift_ist requires paranoid=1"
        .endif

        ASM_CLAC

        .if \has_error_code == 0
        pushq   $-1                             /* ORIG_RAX: no syscall to restart */
        .endif

        /* Save all registers in pt_regs */
        PUSH_AND_CLEAR_REGS
        ENCODE_FRAME_POINTER

        .if \paranoid < 2
        testb   $3, CS(%rsp)                    /* If coming from userspace, switch stacks */
        jnz     .Lfrom_usermode_switch_stack_\@
        .endif

        .if \paranoid
        call    paranoid_entry
        .else
        call    error_entry
        .endif
        UNWIND_HINT_REGS
        /* returned flag: ebx=0: need swapgs on exit, ebx=1: don't need it */

        .if \paranoid
        .if \shift_ist != -1
        TRACE_IRQS_OFF_DEBUG                    /* reload IDT in case of recursion */
        .else
        TRACE_IRQS_OFF
        .endif
        .endif

        movq    %rsp, %rdi                      /* pt_regs pointer */

        .if \has_error_code
        movq    ORIG_RAX(%rsp), %rsi            /* get error code */
        movq    $-1, ORIG_RAX(%rsp)             /* no syscall to restart */
        .else
        xorl    %esi, %esi                      /* no error code */
        .endif

        .if \shift_ist != -1
        subq    $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
        .endif

        call    \do_sym

        .if \shift_ist != -1
        addq    $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
        .endif

        /* these procedures expect "no swapgs" flag in ebx */
        .if \paranoid
        jmp     paranoid_exit
        .else
        jmp     error_exit
        .endif

        .if \paranoid < 2
        /*
         * Entry from userspace.  Switch stacks and treat it
         * as a normal entry.  This means that paranoid handlers
         * run in real process context if user_mode(regs).
         */
.Lfrom_usermode_switch_stack_\@:
        call    error_entry

        movq    %rsp, %rdi                      /* pt_regs pointer */

        .if \has_error_code
        movq    ORIG_RAX(%rsp), %rsi            /* get error code */
        movq    $-1, ORIG_RAX(%rsp)             /* no syscall to restart */
        .else
        xorl    %esi, %esi                      /* no error code */
        .endif

        call    \do_sym

        jmp     error_exit                      /* %ebx: no swapgs flag */
        .endif
END(\sym)
.endm

idtentry divide_error                   do_divide_error                 has_error_code=0
idtentry overflow                       do_overflow                     has_error_code=0
idtentry bounds                         do_bounds                       has_error_code=0
idtentry invalid_op                     do_invalid_op                   has_error_code=0
idtentry device_not_available           do_device_not_available         has_error_code=0
idtentry double_fault                   do_double_fault                 has_error_code=1 paranoid=2
idtentry coprocessor_segment_overrun    do_coprocessor_segment_overrun  has_error_code=0
idtentry invalid_TSS                    do_invalid_TSS                  has_error_code=1
idtentry segment_not_present            do_segment_not_present          has_error_code=1
idtentry spurious_interrupt_bug         do_spurious_interrupt_bug       has_error_code=0
idtentry coprocessor_error              do_coprocessor_error            has_error_code=0
idtentry alignment_check                do_alignment_check              has_error_code=1
idtentry simd_coprocessor_error         do_simd_coprocessor_error       has_error_code=0


        /*
         * Reload gs selector with exception handling
         * edi:  new selector
         */
ENTRY(native_load_gs_index)
        FRAME_BEGIN
        pushfq
        DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI)
        TRACE_IRQS_OFF
        SWAPGS
.Lgs_change:
        movl    %edi, %gs
2:      ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
        SWAPGS
        TRACE_IRQS_FLAGS (%rsp)
        popfq
        FRAME_END
        ret
ENDPROC(native_load_gs_index)
EXPORT_SYMBOL(native_load_gs_index)

        _ASM_EXTABLE(.Lgs_change, bad_gs)
        .section .fixup, "ax"
        /* running with kernelgs */
bad_gs:
        SWAPGS                                  /* switch back to user gs */
.macro ZAP_GS
        /* This can't be a string because the preprocessor needs to see it. */
        movl $__USER_DS, %eax
        movl %eax, %gs
.endm
        ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
        xorl    %eax, %eax
        movl    %eax, %gs
        jmp     2b
        .previous

/* Call softirq on interrupt stack. Interrupts are off. */
ENTRY(do_softirq_own_stack)
        pushq   %rbp
        mov     %rsp, %rbp
        ENTER_IRQ_STACK regs=0 old_rsp=%r11
        call    __do_softirq
        LEAVE_IRQ_STACK regs=0
        leaveq
        ret
ENDPROC(do_softirq_own_stack)

#ifdef CONFIG_XEN
idtentry hypervisor_callback xen_do_hypervisor_callback has_error_code=0

/*
 * A note on the "critical region" in our callback handler.
 * We want to avoid stacking callback handlers due to events occurring
 * during handling of the last event. To do this, we keep events disabled
 * until we've done all processing. HOWEVER, we must enable events before
 * popping the stack frame (can't be done atomically) and so it would still
 * be possible to get enough handler activations to overflow the stack.
 * Although unlikely, bugs of that kind are hard to track down, so we'd
 * like to avoid the possibility.
 * So, on entry to the handler we detect whether we interrupted an
 * existing activation in its critical region -- if so, we pop the current
 * activation and restart the handler using the previous one.
 */
ENTRY(xen_do_hypervisor_callback)               /* do_hypervisor_callback(struct *pt_regs) */

/*
 * Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
 * see the correct pointer to the pt_regs
 */
        UNWIND_HINT_FUNC
        movq    %rdi, %rsp                      /* we don't return, adjust the stack frame */
        UNWIND_HINT_REGS

        ENTER_IRQ_STACK old_rsp=%r10
        call    xen_evtchn_do_upcall
        LEAVE_IRQ_STACK

#ifndef CONFIG_PREEMPT
        call    xen_maybe_preempt_hcall
#endif
        jmp     error_exit
END(xen_do_hypervisor_callback)

/*
 * Hypervisor uses this for application faults while it executes.
 * We get here for two reasons:
 *  1. Fault while reloading DS, ES, FS or GS
 *  2. Fault while executing IRET
 * Category 1 we do not need to fix up as Xen has already reloaded all segment
 * registers that could be reloaded and zeroed the others.
 * Category 2 we fix up by killing the current process. We cannot use the
 * normal Linux return path in this case because if we use the IRET hypercall
 * to pop the stack frame we end up in an infinite loop of failsafe callbacks.
 * We distinguish between categories by comparing each saved segment register
 * with its current contents: any discrepancy means we in category 1.
 */
ENTRY(xen_failsafe_callback)
        UNWIND_HINT_EMPTY
        movl    %ds, %ecx
        cmpw    %cx, 0x10(%rsp)
        jne     1f
        movl    %es, %ecx
        cmpw    %cx, 0x18(%rsp)
        jne     1f
        movl    %fs, %ecx
        cmpw    %cx, 0x20(%rsp)
        jne     1f
        movl    %gs, %ecx
        cmpw    %cx, 0x28(%rsp)
        jne     1f
        /* All segments match their saved values => Category 2 (Bad IRET). */
        movq    (%rsp), %rcx
        movq    8(%rsp), %r11
        addq    $0x30, %rsp
        pushq   $0                              /* RIP */
        UNWIND_HINT_IRET_REGS offset=8
        jmp     general_protection
1:      /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
        movq    (%rsp), %rcx
        movq    8(%rsp), %r11
        addq    $0x30, %rsp
        UNWIND_HINT_IRET_REGS
        pushq   $-1 /* orig_ax = -1 => not a system call */
        PUSH_AND_CLEAR_REGS
        ENCODE_FRAME_POINTER
        jmp     error_exit
END(xen_failsafe_callback)

apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
        xen_hvm_callback_vector xen_evtchn_do_upcall

#endif /* CONFIG_XEN */

#if IS_ENABLED(CONFIG_HYPERV)
apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
        hyperv_callback_vector hyperv_vector_handler
#endif /* CONFIG_HYPERV */

idtentry debug                  do_debug                has_error_code=0        paranoid=1 shift_ist=DEBUG_STACK
idtentry int3                   do_int3                 has_error_code=0        paranoid=1 shift_ist=DEBUG_STACK
idtentry stack_segment          do_stack_segment        has_error_code=1

#ifdef CONFIG_XEN
idtentry xennmi                 do_nmi                  has_error_code=0
idtentry xendebug               do_debug                has_error_code=0
idtentry xenint3                do_int3                 has_error_code=0
#endif

idtentry general_protection     do_general_protection   has_error_code=1
idtentry page_fault             do_page_fault           has_error_code=1

#ifdef CONFIG_KVM_GUEST
idtentry async_page_fault       do_async_page_fault     has_error_code=1
#endif

#ifdef CONFIG_X86_MCE
idtentry machine_check          do_mce                  has_error_code=0        paranoid=1
#endif

/*
 * Switch gs if needed.
 * Use slow, but surefire "are we in kernel?" check.
 * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
 */
ENTRY(paranoid_entry)
        UNWIND_HINT_FUNC
        cld
        movl    $1, %ebx
        movl    $MSR_GS_BASE, %ecx
        rdmsr
        testl   %edx, %edx
        js      1f                              /* negative -> in kernel */
        SWAPGS
        xorl    %ebx, %ebx

1:
        SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14

        ret
END(paranoid_entry)

/*
 * "Paranoid" exit path from exception stack.  This is invoked
 * only on return from non-NMI IST interrupts that came
 * from kernel space.
 *
 * We may be returning to very strange contexts (e.g. very early
 * in syscall entry), so checking for preemption here would
 * be complicated.  Fortunately, we there's no good reason
 * to try to handle preemption here.
 *
 * On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it)
 */
ENTRY(paranoid_exit)
        UNWIND_HINT_REGS
        DISABLE_INTERRUPTS(CLBR_ANY)
        TRACE_IRQS_OFF_DEBUG
        testl   %ebx, %ebx                      /* swapgs needed? */
        jnz     .Lparanoid_exit_no_swapgs
        TRACE_IRQS_IRETQ
        RESTORE_CR3     scratch_reg=%rbx save_reg=%r14
        SWAPGS_UNSAFE_STACK
        jmp     .Lparanoid_exit_restore
.Lparanoid_exit_no_swapgs:
        TRACE_IRQS_IRETQ_DEBUG
.Lparanoid_exit_restore:
        jmp restore_regs_and_return_to_kernel
END(paranoid_exit)

/*
 * Switch gs if needed.
 * Return: EBX=0: came from user mode; EBX=1: otherwise
 */
ENTRY(error_entry)
        UNWIND_HINT_REGS offset=8
        cld
        testb   $3, CS+8(%rsp)
        jz      .Lerror_kernelspace

        /*
         * We entered from user mode or we're pretending to have entered
         * from user mode due to an IRET fault.
         */
        SWAPGS
        /* We have user CR3.  Change to kernel CR3. */
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rax

.Lerror_entry_from_usermode_after_swapgs:
        /* Put us onto the real thread stack. */
        popq    %r12                            /* save return addr in %12 */
        movq    %rsp, %rdi                      /* arg0 = pt_regs pointer */
        call    sync_regs
        movq    %rax, %rsp                      /* switch stack */
        ENCODE_FRAME_POINTER
        pushq   %r12

        /*
         * We need to tell lockdep that IRQs are off.  We can't do this until
         * we fix gsbase, and we should do it before enter_from_user_mode
         * (which can take locks).
         */
        TRACE_IRQS_OFF
        CALL_enter_from_user_mode
        ret

.Lerror_entry_done:
        TRACE_IRQS_OFF
        ret

        /*
         * There are two places in the kernel that can potentially fault with
         * usergs. Handle them here.  B stepping K8s sometimes report a
         * truncated RIP for IRET exceptions returning to compat mode. Check
         * for these here too.
         */
.Lerror_kernelspace:
        incl    %ebx
        leaq    native_irq_return_iret(%rip), %rcx
        cmpq    %rcx, RIP+8(%rsp)
        je      .Lerror_bad_iret
        movl    %ecx, %eax                      /* zero extend */
        cmpq    %rax, RIP+8(%rsp)
        je      .Lbstep_iret
        cmpq    $.Lgs_change, RIP+8(%rsp)
        jne     .Lerror_entry_done

        /*
         * hack: .Lgs_change can fail with user gsbase.  If this happens, fix up
         * gsbase and proceed.  We'll fix up the exception and land in
         * .Lgs_change's error handler with kernel gsbase.
         */
        SWAPGS
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
        jmp .Lerror_entry_done

.Lbstep_iret:
        /* Fix truncated RIP */
        movq    %rcx, RIP+8(%rsp)
        /* fall through */

.Lerror_bad_iret:
        /*
         * We came from an IRET to user mode, so we have user
         * gsbase and CR3.  Switch to kernel gsbase and CR3:
         */
        SWAPGS
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rax

        /*
         * Pretend that the exception came from user mode: set up pt_regs
         * as if we faulted immediately after IRET and clear EBX so that
         * error_exit knows that we will be returning to user mode.
         */
        mov     %rsp, %rdi
        call    fixup_bad_iret
        mov     %rax, %rsp
        decl    %ebx
        jmp     .Lerror_entry_from_usermode_after_swapgs
END(error_entry)


/*
 * On entry, EBX is a "return to kernel mode" flag:
 *   1: already in kernel mode, don't need SWAPGS
 *   0: user gsbase is loaded, we need SWAPGS and standard preparation for return to usermode
 */
ENTRY(error_exit)
        UNWIND_HINT_REGS
        DISABLE_INTERRUPTS(CLBR_ANY)
        TRACE_IRQS_OFF
        testl   %ebx, %ebx
        jnz     retint_kernel
        jmp     retint_user
END(error_exit)

/*
 * Runs on exception stack.  Xen PV does not go through this path at all,
 * so we can use real assembly here.
 *
 * Registers:
 *      %r14: Used to save/restore the CR3 of the interrupted context
 *            when PAGE_TABLE_ISOLATION is in use.  Do not clobber.
 */
ENTRY(nmi)
        UNWIND_HINT_IRET_REGS

        /*
         * We allow breakpoints in NMIs. If a breakpoint occurs, then
         * the iretq it performs will take us out of NMI context.
         * This means that we can have nested NMIs where the next
         * NMI is using the top of the stack of the previous NMI. We
         * can't let it execute because the nested NMI will corrupt the
         * stack of the previous NMI. NMI handlers are not re-entrant
         * anyway.
         *
         * To handle this case we do the following:
         *  Check the a special location on the stack that contains
         *  a variable that is set when NMIs are executing.
         *  The interrupted task's stack is also checked to see if it
         *  is an NMI stack.
         *  If the variable is not set and the stack is not the NMI
         *  stack then:
         *    o Set the special variable on the stack
         *    o Copy the interrupt frame into an "outermost" location on the
         *      stack
         *    o Copy the interrupt frame into an "iret" location on the stack
         *    o Continue processing the NMI
         *  If the variable is set or the previous stack is the NMI stack:
         *    o Modify the "iret" location to jump to the repeat_nmi
         *    o return back to the first NMI
         *
         * Now on exit of the first NMI, we first clear the stack variable
         * The NMI stack will tell any nested NMIs at that point that it is
         * nested. Then we pop the stack normally with iret, and if there was
         * a nested NMI that updated the copy interrupt stack frame, a
         * jump will be made to the repeat_nmi code that will handle the second
         * NMI.
         *
         * However, espfix prevents us from directly returning to userspace
         * with a single IRET instruction.  Similarly, IRET to user mode
         * can fault.  We therefore handle NMIs from user space like
         * other IST entries.
         */

        ASM_CLAC

        /* Use %rdx as our temp variable throughout */
        pushq   %rdx

        testb   $3, CS-RIP+8(%rsp)
        jz      .Lnmi_from_kernel

        /*
         * NMI from user mode.  We need to run on the thread stack, but we
         * can't go through the normal entry paths: NMIs are masked, and
         * we don't want to enable interrupts, because then we'll end
         * up in an awkward situation in which IRQs are on but NMIs
         * are off.
         *
         * We also must not push anything to the stack before switching
         * stacks lest we corrupt the "NMI executing" variable.
         */

        swapgs
        cld
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
        movq    %rsp, %rdx
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
        UNWIND_HINT_IRET_REGS base=%rdx offset=8
        pushq   5*8(%rdx)       /* pt_regs->ss */
        pushq   4*8(%rdx)       /* pt_regs->rsp */
        pushq   3*8(%rdx)       /* pt_regs->flags */
        pushq   2*8(%rdx)       /* pt_regs->cs */
        pushq   1*8(%rdx)       /* pt_regs->rip */
        UNWIND_HINT_IRET_REGS
        pushq   $-1             /* pt_regs->orig_ax */
        PUSH_AND_CLEAR_REGS rdx=(%rdx)
        ENCODE_FRAME_POINTER

        /*
         * At this point we no longer need to worry about stack damage
         * due to nesting -- we're on the normal thread stack and we're
         * done with the NMI stack.
         */

        movq    %rsp, %rdi
        movq    $-1, %rsi
        call    do_nmi

        /*
         * Return back to user mode.  We must *not* do the normal exit
         * work, because we don't want to enable interrupts.
         */
        jmp     swapgs_restore_regs_and_return_to_usermode

.Lnmi_from_kernel:
        /*
         * Here's what our stack frame will look like:
         * +---------------------------------------------------------+
         * | original SS                                             |
         * | original Return RSP                                     |
         * | original RFLAGS                                         |
         * | original CS                                             |
         * | original RIP                                            |
         * +---------------------------------------------------------+
         * | temp storage for rdx                                    |
         * +---------------------------------------------------------+
         * | "NMI executing" variable                                |
         * +---------------------------------------------------------+
         * | iret SS          } Copied from "outermost" frame        |
         * | iret Return RSP  } on each loop iteration; overwritten  |
         * | iret RFLAGS      } by a nested NMI to force another     |
         * | iret CS          } iteration if needed.                 |
         * | iret RIP         }                                      |
         * +---------------------------------------------------------+
         * | outermost SS          } initialized in first_nmi;       |
         * | outermost Return RSP  } will not be changed before      |
         * | outermost RFLAGS      } NMI processing is done.         |
         * | outermost CS          } Copied to "iret" frame on each  |
         * | outermost RIP         } iteration.                      |
         * +---------------------------------------------------------+
         * | pt_regs                                                 |
         * +---------------------------------------------------------+
         *
         * The "original" frame is used by hardware.  Before re-enabling
         * NMIs, we need to be done with it, and we need to leave enough
         * space for the asm code here.
         *
         * We return by executing IRET while RSP points to the "iret" frame.
         * That will either return for real or it will loop back into NMI
         * processing.
         *
         * The "outermost" frame is copied to the "iret" frame on each
         * iteration of the loop, so each iteration starts with the "iret"
         * frame pointing to the final return target.
         */

        /*
         * Determine whether we're a nested NMI.
         *
         * If we interrupted kernel code between repeat_nmi and
         * end_repeat_nmi, then we are a nested NMI.  We must not
         * modify the "iret" frame because it's being written by
         * the outer NMI.  That's okay; the outer NMI handler is
         * about to about to call do_nmi anyway, so we can just
         * resume the outer NMI.
         */

        movq    $repeat_nmi, %rdx
        cmpq    8(%rsp), %rdx
        ja      1f
        movq    $end_repeat_nmi, %rdx
        cmpq    8(%rsp), %rdx
        ja      nested_nmi_out
1:

        /*
         * Now check "NMI executing".  If it's set, then we're nested.
         * This will not detect if we interrupted an outer NMI just
         * before IRET.
         */
        cmpl    $1, -8(%rsp)
        je      nested_nmi

        /*
         * Now test if the previous stack was an NMI stack.  This covers
         * the case where we interrupt an outer NMI after it clears
         * "NMI executing" but before IRET.  We need to be careful, though:
         * there is one case in which RSP could point to the NMI stack
         * despite there being no NMI active: naughty userspace controls
         * RSP at the very beginning of the SYSCALL targets.  We can
         * pull a fast one on naughty userspace, though: we program
         * SYSCALL to mask DF, so userspace cannot cause DF to be set
         * if it controls the kernel's RSP.  We set DF before we clear
         * "NMI executing".
         */
        lea     6*8(%rsp), %rdx
        /* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
        cmpq    %rdx, 4*8(%rsp)
        /* If the stack pointer is above the NMI stack, this is a normal NMI */
        ja      first_nmi

        subq    $EXCEPTION_STKSZ, %rdx
        cmpq    %rdx, 4*8(%rsp)
        /* If it is below the NMI stack, it is a normal NMI */
        jb      first_nmi

        /* Ah, it is within the NMI stack. */

        testb   $(X86_EFLAGS_DF >> 8), (3*8 + 1)(%rsp)
        jz      first_nmi       /* RSP was user controlled. */

        /* This is a nested NMI. */

nested_nmi:
        /*
         * Modify the "iret" frame to point to repeat_nmi, forcing another
         * iteration of NMI handling.
         */
        subq    $8, %rsp
        leaq    -10*8(%rsp), %rdx
        pushq   $__KERNEL_DS
        pushq   %rdx
        pushfq
        pushq   $__KERNEL_CS
        pushq   $repeat_nmi

        /* Put stack back */
        addq    $(6*8), %rsp

nested_nmi_out:
        popq    %rdx

        /* We are returning to kernel mode, so this cannot result in a fault. */
        iretq

first_nmi:
        /* Restore rdx. */
        movq    (%rsp), %rdx

        /* Make room for "NMI executing". */
        pushq   $0

        /* Leave room for the "iret" frame */
        subq    $(5*8), %rsp

        /* Copy the "original" frame to the "outermost" frame */
        .rept 5
        pushq   11*8(%rsp)
        .endr
        UNWIND_HINT_IRET_REGS

        /* Everything up to here is safe from nested NMIs */

#ifdef CONFIG_DEBUG_ENTRY
        /*
         * For ease of testing, unmask NMIs right away.  Disabled by
         * default because IRET is very expensive.
         */
        pushq   $0              /* SS */
        pushq   %rsp            /* RSP (minus 8 because of the previous push) */
        addq    $8, (%rsp)      /* Fix up RSP */
        pushfq                  /* RFLAGS */
        pushq   $__KERNEL_CS    /* CS */
        pushq   $1f             /* RIP */
        iretq                   /* continues at repeat_nmi below */
        UNWIND_HINT_IRET_REGS
1:
#endif

repeat_nmi:
        /*
         * If there was a nested NMI, the first NMI's iret will return
         * here. But NMIs are still enabled and we can take another
         * nested NMI. The nested NMI checks the interrupted RIP to see
         * if it is between repeat_nmi and end_repeat_nmi, and if so
         * it will just return, as we are about to repeat an NMI anyway.
         * This makes it safe to copy to the stack frame that a nested
         * NMI will update.
         *
         * RSP is pointing to "outermost RIP".  gsbase is unknown, but, if
         * we're repeating an NMI, gsbase has the same value that it had on
         * the first iteration.  paranoid_entry will load the kernel
         * gsbase if needed before we call do_nmi.  "NMI executing"
         * is zero.
         */
        movq    $1, 10*8(%rsp)          /* Set "NMI executing". */

        /*
         * Copy the "outermost" frame to the "iret" frame.  NMIs that nest
         * here must not modify the "iret" frame while we're writing to
         * it or it will end up containing garbage.
         */
        addq    $(10*8), %rsp
        .rept 5
        pushq   -6*8(%rsp)
        .endr
        subq    $(5*8), %rsp
end_repeat_nmi:

        /*
         * Everything below this point can be preempted by a nested NMI.
         * If this happens, then the inner NMI will change the "iret"
         * frame to point back to repeat_nmi.
         */
        pushq   $-1                             /* ORIG_RAX: no syscall to restart */
        PUSH_AND_CLEAR_REGS
        ENCODE_FRAME_POINTER

        /*
         * Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
         * as we should not be calling schedule in NMI context.
         * Even with normal interrupts enabled. An NMI should not be
         * setting NEED_RESCHED or anything that normal interrupts and
         * exceptions might do.
         */
        call    paranoid_entry
        UNWIND_HINT_REGS

        /* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */
        movq    %rsp, %rdi
        movq    $-1, %rsi
        call    do_nmi

        RESTORE_CR3 scratch_reg=%r15 save_reg=%r14

        testl   %ebx, %ebx                      /* swapgs needed? */
        jnz     nmi_restore
nmi_swapgs:
        SWAPGS_UNSAFE_STACK
nmi_restore:
        POP_REGS

        /*
         * Skip orig_ax and the "outermost" frame to point RSP at the "iret"
         * at the "iret" frame.
         */
        addq    $6*8, %rsp

        /*
         * Clear "NMI executing".  Set DF first so that we can easily
         * distinguish the remaining code between here and IRET from
         * the SYSCALL entry and exit paths.
         *
         * We arguably should just inspect RIP instead, but I (Andy) wrote
         * this code when I had the misapprehension that Xen PV supported
         * NMIs, and Xen PV would break that approach.
         */
        std
        movq    $0, 5*8(%rsp)           /* clear "NMI executing" */

        /*
         * iretq reads the "iret" frame and exits the NMI stack in a
         * single instruction.  We are returning to kernel mode, so this
         * cannot result in a fault.  Similarly, we don't need to worry
         * about espfix64 on the way back to kernel mode.
         */
        iretq
END(nmi)

ENTRY(ignore_sysret)
        UNWIND_HINT_EMPTY
        mov     $-ENOSYS, %eax
        sysret
END(ignore_sysret)

ENTRY(rewind_stack_do_exit)
        UNWIND_HINT_FUNC
        /* Prevent any naive code from trying to unwind to our caller. */
        xorl    %ebp, %ebp

        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rax
        leaq    -PTREGS_SIZE(%rax), %rsp
        UNWIND_HINT_FUNC sp_offset=PTREGS_SIZE

        call    do_exit
END(rewind_stack_do_exit)