[PATCH] arm64: mte: avoid clearing PSTATE.TCO on entry unless necessary

[Peter Collingbourne]

On some microarchitectures, clearing PSTATE.TCO is expensive. Clearing
TCO is only necessary if in-kernel MTE is enabled, or if MTE is
enabled in the userspace process in synchronous (or, soon, asymmetric)
mode, because we do not report uaccess faults to userspace in none
or asynchronous modes. Therefore, adjust the kernel entry code to
clear TCO only if necessary.

Signed-off-by: Peter Collingbourne <pcc at google.com>
Link: https://linux-review.googlesource.com/id/I52d82a580bd0500d420be501af2c35fa8c90729e
---
 arch/arm64/kernel/entry.S | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 2f69ae43941d..85ead6bbb38e 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -269,7 +269,28 @@ alternative_else_nop_endif
 	.else
 	add	x21, sp, #PT_REGS_SIZE
 	get_current_task tsk
+	ldr	x0, [tsk, THREAD_SCTLR_USER]
 	.endif /* \el == 0 */
+
+	/*
+	 * Re-enable tag checking (TCO set on exception entry). This is only
+	 * necessary if MTE is enabled in either the kernel or the userspace
+	 * task in synchronous mode. With MTE disabled in the kernel and
+	 * disabled or asynchronous in userspace, tag check faults (including in
+	 * uaccesses) are not reported, therefore there is no need to re-enable
+	 * checking. This is beneficial on microarchitectures where re-enabling
+	 * TCO is expensive.
+	 */
+#ifdef CONFIG_ARM64_MTE
+alternative_cb	kasan_hw_tags_enable
+	tbz	x0, #SCTLR_EL1_TCF0_SHIFT, 1f
+alternative_cb_end
+alternative_if ARM64_MTE
+	SET_PSTATE_TCO(0)
+alternative_else_nop_endif
+1:
+#endif
+
 	mrs	x22, elr_el1
 	mrs	x23, spsr_el1
 	stp	lr, x21, [sp, #S_LR]
@@ -308,13 +329,6 @@ alternative_if ARM64_HAS_IRQ_PRIO_MASKING
 	msr_s	SYS_ICC_PMR_EL1, x20
 alternative_else_nop_endif
 
-	/* Re-enable tag checking (TCO set on exception entry) */
-#ifdef CONFIG_ARM64_MTE
-alternative_if ARM64_MTE
-	SET_PSTATE_TCO(0)
-alternative_else_nop_endif
-#endif
-
 	/*
 	 * Registers that may be useful after this macro is invoked:
 	 *
-- 
2.33.0.800.g4c38ced690-goog