From a4e5927115f30a301f9939ed43e6a21a343e06ad Mon Sep 17 00:00:00 2001 From: Carl Worth Date: Thu, 15 Jan 2026 15:07:18 -0800 Subject: [PATCH] arm64: mte: Set TCMA1 whenever MTE is present in the kernel Set the TCMA1 bit so that access to TTBR1 addresses with 0xf in their tag bits will be treated as tag unchecked. This is important to avoid unwanted tag checking on some systems. Specifically, SCTLR_EL1.TCF can be set to indicate that no tag check faults are desired. But the architecture doesn't guarantee that in this case the system won't still perform tag checks. Use TCMA1 to ensure that undesired tag checks are not performed. This bit was already set in the KASAN case. Adding it to the non-KASAN case prevents tag checking since all TTBR1 address will have a value of 0xf in their tag bits. This patch has been measured on an Ampere system to improve the following: * Eliminate over 98% of kernel-side tag checks during "perf bench futex hash", as measured with "perf stat". * Eliminate all MTE overhead (was previously a 25% performance penalty) from the Phoronix pts/memcached benchmark (1:10 Set:Get ration with 96 cores). Reported-by: Taehyun Noh Suggested-by: Catalin Marinas Signed-off-by: Carl Worth Reviewed-by: Catalin Marinas Signed-off-by: Will Deacon --- arch/arm64/mm/proc.S | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S index 01e8681164488..680ff21d79f18 100644 --- a/arch/arm64/mm/proc.S +++ b/arch/arm64/mm/proc.S @@ -48,14 +48,14 @@ #define TCR_KASAN_SW_FLAGS 0 #endif -#ifdef CONFIG_KASAN_HW_TAGS -#define TCR_MTE_FLAGS TCR_EL1_TCMA1 | TCR_EL1_TBI1 | TCR_EL1_TBID1 -#elif defined(CONFIG_ARM64_MTE) +#ifdef CONFIG_ARM64_MTE /* * The mte_zero_clear_page_tags() implementation uses DC GZVA, which relies on - * TBI being enabled at EL1. + * TBI being enabled at EL1. TCMA1 is needed to treat accesses with the + * match-all tag (0xF) as Tag Unchecked, irrespective of the SCTLR_EL1.TCF + * setting. */ -#define TCR_MTE_FLAGS TCR_EL1_TBI1 | TCR_EL1_TBID1 +#define TCR_MTE_FLAGS TCR_EL1_TCMA1 | TCR_EL1_TBI1 | TCR_EL1_TBID1 #else #define TCR_MTE_FLAGS 0 #endif -- 2.47.3