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 <redacted>
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
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