[PATCH] arm64: mte: Make mte_check_tfsr_*() conditional on KASAN instead of MTE

[Peter Collingbourne]

The check in mte_check_tfsr_el1() is only necessary if HW tag
based KASAN is enabled. However, we were also executing the check
if MTE is enabled and KASAN is enabled at build time but disabled
at runtime. This turned out to cause a measurable increase in
power consumption on a specific microarchitecture after enabling
MTE. Moreover, on the same system, an increase in invalid syscall
latency (as measured by [1]) of around 20-30% (depending on the
cluster) was observed after enabling MTE; this almost entirely goes
away after removing this check. Therefore, make the check conditional
on whether KASAN is enabled rather than on whether MTE is enabled.

[1] https://lore.kernel.org/all/CAMn1gO4MwRV8bmFJ_SeY5tsYNPn2ZP56LjAhafygjFaKuu5ouw@mail.gmail.com/

Signed-off-by: Peter Collingbourne <pcc at google.com>
Link: https://linux-review.googlesource.com/id/I22d98d1483dd400a95595946552b769a5a1ad7bd
---
 arch/arm64/include/asm/mte.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index 91fbd5c8a3911..0f84518632b4a 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -182,7 +182,7 @@ void mte_check_tfsr_el1(void);
 
 static inline void mte_check_tfsr_entry(void)
 {
-	if (!system_supports_mte())
+	if (!kasan_hw_tags_enabled())
 		return;
 
 	mte_check_tfsr_el1();
@@ -190,7 +190,7 @@ static inline void mte_check_tfsr_entry(void)
 
 static inline void mte_check_tfsr_exit(void)
 {
-	if (!system_supports_mte())
+	if (!kasan_hw_tags_enabled())
 		return;
 
 	/*
-- 
2.45.1.288.g0e0cd299f1-goog