arm64: add MTE supported check to thread switching and syscall entry/exit

This lets us avoid doing unnecessary work on hardware that does not
support MTE, and will allow us to freely use MTE instructions in the
code called by mte_thread_switch().

Since this would mean that we do a redundant check in
mte_check_tfsr_el1(), remove it and add two checks now required in its
callers. This also avoids an unnecessary DSB+ISB sequence on the syscall
exit path for hardware not supporting MTE.

Fixes: 65812c6921cc ("arm64: mte: Enable async tag check fault")
Cc: <[e-mail]> # 5.13.x
Signed-off-by: Peter Collingbourne <[e-mail]>
Link: https://linux-review.googlesource.com/id/I02fd000d1ef2c86c7d2952a7f099b254ec227a5d
Link: https://lore.kernel.org/r/[e-mail]
[[e-mail]: adjust the commit log slightly]
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/include/asm/mte.h

···
 
 static inline void mte_check_tfsr_entry(void)
 {
+	if (!system_supports_mte())
+		return;
+
 	mte_check_tfsr_el1();
 }
 
 static inline void mte_check_tfsr_exit(void)
 {
+	if (!system_supports_mte())
+		return;
+
 	/*
 	 * The asynchronous faults are sync'ed automatically with
 	 * TFSR_EL1 on kernel entry but for exit an explicit dsb()

arch/arm64/kernel/mte.c

···
 #ifdef CONFIG_KASAN_HW_TAGS
 void mte_check_tfsr_el1(void)
 {
-	u64 tfsr_el1;
-
-	if (!system_supports_mte())
-		return;
-
-	tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
+	u64 tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
 
 	if (unlikely(tfsr_el1 & SYS_TFSR_EL1_TF1)) {
 		/*
···
 
 void mte_thread_switch(struct task_struct *next)
 {
+	if (!system_supports_mte())
+		return;
+
 	mte_update_sctlr_user(next);
 
 	/*