KVM: PPC: Book3S HV: Snapshot timebase offset on guest entry

Currently, the HV KVM guest entry/exit code adds the timebase offset
from the vcore struct to the timebase on guest entry, and subtracts
it on guest exit.  Which is fine, except that it is possible for
userspace to change the offset using the SET_ONE_REG interface while
the vcore is running, as there is only one timebase offset per vcore
but potentially multiple VCPUs in the vcore.  If that were to happen,
KVM would subtract a different offset on guest exit from that which
it had added on guest entry, leading to the timebase being out of sync
between cores in the host, which then leads to bad things happening
such as hangs and spurious watchdog timeouts.

To fix this, we add a new field 'tb_offset_applied' to the vcore struct
which stores the offset that is currently applied to the timebase.
This value is set from the vcore tb_offset field on guest entry, and
is what is subtracted from the timebase on guest exit.  Since it is
zero when the timebase offset is not applied, we can simplify the
logic in kvmhv_start_timing and kvmhv_accumulate_time.

In addition, we had secondary threads reading the timebase while
running concurrently with code on the primary thread which would
eventually add or subtract the timebase offset from the timebase.
This occurred while saving or restoring the DEC register value on
the secondary threads.  Although no specific incorrect behaviour has
been observed, this is a race which should be fixed.  To fix it, we
move the DEC saving code to just before we call kvmhv_commence_exit,
and the DEC restoring code to after the point where we have waited
for the primary thread to switch the MMU context and add the timebase
offset.  That way we are sure that the timebase contains the guest
timebase value in both cases.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
index 4c02a7378d067e6dd5afc12b7336f90353879abc..e7377b73cfecaa2874fe240ee861e472cfa9309d 100644 (file)
--- a/arch/powerpc/include/asm/kvm_book3s.h
+++ b/arch/powerpc/include/asm/kvm_book3s.h
@@ -96,6 +96,7 @@ struct kvmppc_vcore {
        struct kvm_vcpu *runner;
        struct kvm *kvm;
        u64 tb_offset;          /* guest timebase - host timebase */
+       u64 tb_offset_applied;  /* timebase offset currently in force */
        ulong lpcr;
        u32 arch_compat;
        ulong pcr;

diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 6bee65f3cfd34bf896ef770f805ec62d417ae9db..373dc1d6ef44e99854200208466f741489944423 100644 (file)
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -562,6 +562,7 @@ int main(void)
        OFFSET(VCORE_NAPPING_THREADS, kvmppc_vcore, napping_threads);
        OFFSET(VCORE_KVM, kvmppc_vcore, kvm);
        OFFSET(VCORE_TB_OFFSET, kvmppc_vcore, tb_offset);
+       OFFSET(VCORE_TB_OFFSET_APPL, kvmppc_vcore, tb_offset_applied);
        OFFSET(VCORE_LPCR, kvmppc_vcore, lpcr);
        OFFSET(VCORE_PCR, kvmppc_vcore, pcr);
        OFFSET(VCORE_DPDES, kvmppc_vcore, dpdes);

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 4d07fca5121c56270aafd6f820009561d399d628..9963f65c212b8cad5a348830651df52b2b7e0c0f 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -2441,6 +2441,7 @@ static void init_vcore_to_run(struct kvmppc_vcore *vc)
        vc->in_guest = 0;
        vc->napping_threads = 0;
        vc->conferring_threads = 0;
+       vc->tb_offset_applied = 0;
 }
 
 static bool can_dynamic_split(struct kvmppc_vcore *vc, struct core_info *cip)

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index bd63fa8a08b5dd3edd0800ac34e483a8501f401c..25c32e421b5771d322b1b51995d08e447389c13d 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -692,6 +692,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
 22:    ld      r8,VCORE_TB_OFFSET(r5)
        cmpdi   r8,0
        beq     37f
+       std     r8, VCORE_TB_OFFSET_APPL(r5)
        mftb    r6              /* current host timebase */
        add     r8,r8,r6
        mtspr   SPRN_TBU40,r8   /* update upper 40 bits */
@@ -940,18 +941,6 @@ FTR_SECTION_ELSE
 ALT_FTR_SECTION_END_IFCLR(CPU_FTR_ARCH_300)
 8:
 
-       /*
-        * Set the decrementer to the guest decrementer.
-        */
-       ld      r8,VCPU_DEC_EXPIRES(r4)
-       /* r8 is a host timebase value here, convert to guest TB */
-       ld      r5,HSTATE_KVM_VCORE(r13)
-       ld      r6,VCORE_TB_OFFSET(r5)
-       add     r8,r8,r6
-       mftb    r7
-       subf    r3,r7,r8
-       mtspr   SPRN_DEC,r3
-
        ld      r5, VCPU_SPRG0(r4)
        ld      r6, VCPU_SPRG1(r4)
        ld      r7, VCPU_SPRG2(r4)
@@ -1005,6 +994,18 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_ARCH_300)
        mtspr   SPRN_LPCR,r8
        isync
 
+       /*
+        * Set the decrementer to the guest decrementer.
+        */
+       ld      r8,VCPU_DEC_EXPIRES(r4)
+       /* r8 is a host timebase value here, convert to guest TB */
+       ld      r5,HSTATE_KVM_VCORE(r13)
+       ld      r6,VCORE_TB_OFFSET_APPL(r5)
+       add     r8,r8,r6
+       mftb    r7
+       subf    r3,r7,r8
+       mtspr   SPRN_DEC,r3
+
        /* Check if HDEC expires soon */
        mfspr   r3, SPRN_HDEC
        EXTEND_HDEC(r3)
@@ -1597,8 +1598,27 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
 
 guest_bypass:
        stw     r12, STACK_SLOT_TRAP(r1)
-       mr      r3, r12
+
+       /* Save DEC */
+       /* Do this before kvmhv_commence_exit so we know TB is guest TB */
+       ld      r3, HSTATE_KVM_VCORE(r13)
+       mfspr   r5,SPRN_DEC
+       mftb    r6
+       /* On P9, if the guest has large decr enabled, don't sign extend */
+BEGIN_FTR_SECTION
+       ld      r4, VCORE_LPCR(r3)
+       andis.  r4, r4, LPCR_LD@h
+       bne     16f
+END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
+       extsw   r5,r5
+16:    add     r5,r5,r6
+       /* r5 is a guest timebase value here, convert to host TB */
+       ld      r4,VCORE_TB_OFFSET_APPL(r3)
+       subf    r5,r4,r5
+       std     r5,VCPU_DEC_EXPIRES(r9)
+
        /* Increment exit count, poke other threads to exit */
+       mr      r3, r12
        bl      kvmhv_commence_exit
        nop
        ld      r9, HSTATE_KVM_VCPU(r13)
@@ -1639,23 +1659,6 @@ guest_bypass:
        mtspr   SPRN_PURR,r3
        mtspr   SPRN_SPURR,r4
 
-       /* Save DEC */
-       ld      r3, HSTATE_KVM_VCORE(r13)
-       mfspr   r5,SPRN_DEC
-       mftb    r6
-       /* On P9, if the guest has large decr enabled, don't sign extend */
-BEGIN_FTR_SECTION
-       ld      r4, VCORE_LPCR(r3)
-       andis.  r4, r4, LPCR_LD@h
-       bne     16f
-END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
-       extsw   r5,r5
-16:    add     r5,r5,r6
-       /* r5 is a guest timebase value here, convert to host TB */
-       ld      r4,VCORE_TB_OFFSET(r3)
-       subf    r5,r4,r5
-       std     r5,VCPU_DEC_EXPIRES(r9)
-
 BEGIN_FTR_SECTION
        b       8f
 END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
@@ -2017,9 +2020,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 
 27:
        /* Subtract timebase offset from timebase */
-       ld      r8,VCORE_TB_OFFSET(r5)
+       ld      r8, VCORE_TB_OFFSET_APPL(r5)
        cmpdi   r8,0
        beq     17f
+       li      r0, 0
+       std     r0, VCORE_TB_OFFSET_APPL(r5)
        mftb    r6                      /* current guest timebase */
        subf    r8,r8,r6
        mtspr   SPRN_TBU40,r8           /* update upper 40 bits */
@@ -2700,7 +2705,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
        add     r3, r3, r5
        ld      r4, HSTATE_KVM_VCPU(r13)
        ld      r5, HSTATE_KVM_VCORE(r13)
-       ld      r6, VCORE_TB_OFFSET(r5)
+       ld      r6, VCORE_TB_OFFSET_APPL(r5)
        subf    r3, r6, r3      /* convert to host TB value */
        std     r3, VCPU_DEC_EXPIRES(r4)
 
@@ -2799,7 +2804,7 @@ END_FTR_SECTION(CPU_FTR_TM | CPU_FTR_P9_TM_HV_ASSIST, 0)
        /* Restore guest decrementer */
        ld      r3, VCPU_DEC_EXPIRES(r4)
        ld      r5, HSTATE_KVM_VCORE(r13)
-       ld      r6, VCORE_TB_OFFSET(r5)
+       ld      r6, VCORE_TB_OFFSET_APPL(r5)
        add     r3, r3, r6      /* convert host TB to guest TB value */
        mftb    r7
        subf    r3, r7, r3
@@ -3606,12 +3611,9 @@ kvmppc_fix_pmao:
  */
 kvmhv_start_timing:
        ld      r5, HSTATE_KVM_VCORE(r13)
-       lbz     r6, VCORE_IN_GUEST(r5)
-       cmpwi   r6, 0
-       beq     5f                              /* if in guest, need to */
-       ld      r6, VCORE_TB_OFFSET(r5)         /* subtract timebase offset */
-5:     mftb    r5
-       subf    r5, r6, r5
+       ld      r6, VCORE_TB_OFFSET_APPL(r5)
+       mftb    r5
+       subf    r5, r6, r5      /* subtract current timebase offset */
        std     r3, VCPU_CUR_ACTIVITY(r4)
        std     r5, VCPU_ACTIVITY_START(r4)
        blr
@@ -3622,15 +3624,12 @@ kvmhv_start_timing:
  */
 kvmhv_accumulate_time:
        ld      r5, HSTATE_KVM_VCORE(r13)
-       lbz     r8, VCORE_IN_GUEST(r5)
-       cmpwi   r8, 0
-       beq     4f                              /* if in guest, need to */
-       ld      r8, VCORE_TB_OFFSET(r5)         /* subtract timebase offset */
-4:     ld      r5, VCPU_CUR_ACTIVITY(r4)
+       ld      r8, VCORE_TB_OFFSET_APPL(r5)
+       ld      r5, VCPU_CUR_ACTIVITY(r4)
        ld      r6, VCPU_ACTIVITY_START(r4)
        std     r3, VCPU_CUR_ACTIVITY(r4)
        mftb    r7
-       subf    r7, r8, r7
+       subf    r7, r8, r7      /* subtract current timebase offset */
        std     r7, VCPU_ACTIVITY_START(r4)
        cmpdi   r5, 0
        beqlr