arm64: mte: optimize asynchronous tag check fault flag check

We don't need to check for MTE support before checking the flag
because it can only be set if the hardware supports MTE. As a result
we can unconditionally check the flag bit which is expected to be in
a register and therefore the check can be done in a single instruction
instead of first needing to load the hwcaps.

On a DragonBoard 845c with a kernel built with CONFIG_ARM64_MTE=y with
the powersave governor this reduces the cost of a kernel entry/exit
(invalid syscall) from 465.1ns to 463.8ns.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/If4dc3501fd4e4f287322f17805509613cfe47d24
Link: https://lore.kernel.org/r/20201118032051.1405907-1-pcc@google.com
[catalin.marinas@arm.com: remove IS_ENABLED(CONFIG_ARM64_MTE)]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index e4c0dadf0d92c8fa71ebe055780dc4f08764a2d6..fe3fa8fd13c8383eeab1f9a879c99d45f43ecb68 100644 (file)
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -123,7 +123,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
        local_daif_restore(DAIF_PROCCTX);
        user_exit();
 
-       if (system_supports_mte() && (flags & _TIF_MTE_ASYNC_FAULT)) {
+       if (flags & _TIF_MTE_ASYNC_FAULT) {
                /*
                 * Process the asynchronous tag check fault before the actual
                 * syscall. do_notify_resume() will send a signal to userspace