[PATCH] arm64: mte: optimize asynchronous tag check fault flag check

[Peter Collingbourne]

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 at google.com>
Link: https://linux-review.googlesource.com/id/If4dc3501fd4e4f287322f17805509613cfe47d24
---
 arch/arm64/kernel/syscall.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index e4c0dadf0d92..f533e83fd722 100644
--- 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 (IS_ENABLED(CONFIG_ARM64_MTE) && (flags & _TIF_MTE_ASYNC_FAULT)) {
 		/*
 		 * Process the asynchronous tag check fault before the actual
 		 * syscall. do_notify_resume() will send a signal to userspace
-- 
2.29.2.299.gdc1121823c-goog