[PATCH v2] arm64: mte: avoid TFSR related operations unless in async mode

[Peter Collingbourne]

There is no reason to touch TFSR nor issue a DSB unless our task is
in asynchronous mode. Since these operations (especially the DSB)
may be expensive on certain microarchitectures, only perform them
if necessary.

Furthermore, stop clearing TFSR on entry because it will be cleared
on exit and it is not necessary to have any particular value in TFSR
between entry and exit.

Signed-off-by: Peter Collingbourne <pcc at google.com>
Link: https://linux-review.googlesource.com/id/Ib353a63e3d0abc2b0b008e96aa2d9692cfc1b815
---
v2:
- avoid clearing TFSR on entry

 arch/arm64/kernel/entry.S | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 863d44f73028..5cf160135411 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -133,29 +133,37 @@ alternative_cb_end
 	.endm
 
 	/* Check for MTE asynchronous tag check faults */
-	.macro check_mte_async_tcf, tmp, ti_flags
+	.macro check_mte_async_tcf, tmp, ti_flags, thread_sctlr
 #ifdef CONFIG_ARM64_MTE
 	.arch_extension lse
 alternative_if_not ARM64_MTE
 	b	1f
 alternative_else_nop_endif
+	/*
+	 * Asynchronous tag check faults are only possible in ASYNC (2) or
+	 * ASYM (3) modes. In each of these modes bit 1 of SCTLR_EL1.TCF0 is
+	 * set, so skip the check if it is unset.
+	 */
+	tbz	\thread_sctlr, #(SCTLR_EL1_TCF0_SHIFT + 1), 1f
 	mrs_s	\tmp, SYS_TFSRE0_EL1
 	tbz	\tmp, #SYS_TFSR_EL1_TF0_SHIFT, 1f
 	/* Asynchronous TCF occurred for TTBR0 access, set the TI flag */
 	mov	\tmp, #_TIF_MTE_ASYNC_FAULT
 	add	\ti_flags, tsk, #TSK_TI_FLAGS
 	stset	\tmp, [\ti_flags]
-	msr_s	SYS_TFSRE0_EL1, xzr
 1:
 #endif
 	.endm
 
 	/* Clear the MTE asynchronous tag check faults */
-	.macro clear_mte_async_tcf
+	.macro clear_mte_async_tcf thread_sctlr
 #ifdef CONFIG_ARM64_MTE
 alternative_if ARM64_MTE
+	/* See comment in check_mte_async_tcf above. */
+	tbz	\thread_sctlr, #(SCTLR_EL1_TCF0_SHIFT + 1), 1f
 	dsb	ish
 	msr_s	SYS_TFSRE0_EL1, xzr
+1:
 alternative_else_nop_endif
 #endif
 	.endm
@@ -231,8 +239,8 @@ alternative_else_nop_endif
 	disable_step_tsk x19, x20
 
 	/* Check for asynchronous tag check faults in user space */
-	check_mte_async_tcf x22, x23
-	apply_ssbd 1, x22, x23
+	ldr	x0, [tsk, THREAD_SCTLR_USER]
+	check_mte_async_tcf x22, x23, x0
 
 #ifdef CONFIG_ARM64_PTR_AUTH
 alternative_if ARM64_HAS_ADDRESS_AUTH
@@ -245,7 +253,6 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
 	 * was disabled on kernel exit then we would have left the kernel IA
 	 * installed so there is no need to install it again.
 	 */
-	ldr	x0, [tsk, THREAD_SCTLR_USER]
 	tbz	x0, SCTLR_ELx_ENIA_SHIFT, 1f
 	__ptrauth_keys_install_kernel_nosync tsk, x20, x22, x23
 	b	2f
@@ -258,6 +265,8 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
 alternative_else_nop_endif
 #endif
 
+	apply_ssbd 1, x22, x23
+
 	mte_set_kernel_gcr x22, x23
 
 	scs_load tsk
@@ -362,6 +371,10 @@ alternative_else_nop_endif
 3:
 	scs_save tsk
 
+	/* Ignore asynchronous tag check faults in the uaccess routines */
+	ldr	x0, [tsk, THREAD_SCTLR_USER]
+	clear_mte_async_tcf x0
+
 #ifdef CONFIG_ARM64_PTR_AUTH
 alternative_if ARM64_HAS_ADDRESS_AUTH
 	/*
@@ -371,7 +384,6 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
 	 *
 	 * No kernel C function calls after this.
 	 */
-	ldr	x0, [tsk, THREAD_SCTLR_USER]
 	tbz	x0, SCTLR_ELx_ENIA_SHIFT, 1f
 	__ptrauth_keys_install_user tsk, x0, x1, x2
 	b	2f
@@ -599,8 +611,6 @@ SYM_CODE_START_LOCAL(ret_to_user)
 	cbnz	x2, work_pending
 finish_ret_to_user:
 	user_enter_irqoff
-	/* Ignore asynchronous tag check faults in the uaccess routines */
-	clear_mte_async_tcf
 	enable_step_tsk x19, x2
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	bl	stackleak_erase
-- 
2.32.0.93.g670b81a890-goog