Re: [PATCH] arm64: mte: Skip TFSR_EL1 checks and barriers in synchronous tag check mode

[Muhammad Usama Anjum]

Please review and share your thoughts.

On 11/03/2026 5:50 pm, Muhammad Usama Anjum wrote:
> In MTE synchronous mode, tag check faults are reported as immediate
> Data Abort exceptions. The TFSR_EL1.TF1 bit is never set, since faults
> never go through the asynchronous path. Therefore, reading TFSR_EL1
> and executing data and instruction barriers on kernel entry, exit,
> context switch, and suspend is unnecessary overhead in sync mode.
> 
> The exit path (mte_check_tfsr_exit) and the assembly paths
> (check_mte_async_tcf / clear_mte_async_tcf in entry.S) already had this
> check. Extend the same optimization on kernel entry/exit, context
> switch and suspend.
> 
> All mte kselftests pass. The kunit before and after the patch show same
> results.
> 
> A selection of test_vmalloc benchmarks running on a arm64 machine.
> v6.19 is the baseline. (>0 is faster, <0 is slower, (R)/(I) =
> statistically significant Regression/Improvement). Based on significance
> and ignoring the noise, the benchmarks improved.
> 
> * 77 result classes were considered, with 9 wins, 0 losses and 68 ties
> 
> Results of fastpath [1] on v6.19 vs this patch:
> 
> +----------------------------+----------------------------------------------------------+------------+
> | Benchmark                  | Result Class                                             |   barriers |
> +============================+==========================================================+============+
> | micromm/fork               | fork: p:1, d:10 (seconds)                                |  (I) 2.75% |
> |                            | fork: p:512, d:10 (seconds)                              |      0.96% |
> +----------------------------+----------------------------------------------------------+------------+
> | micromm/munmap             | munmap: p:1, d:10 (seconds)                              |     -1.78% |
> |                            | munmap: p:512, d:10 (seconds)                            |      5.02% |
> +----------------------------+----------------------------------------------------------+------------+
> | micromm/vmalloc            | fix_align_alloc_test: p:1, h:0, l:500000 (usec)          |     -0.56% |
> |                            | fix_size_alloc_test: p:1, h:0, l:500000 (usec)           |      0.70% |
> |                            | fix_size_alloc_test: p:4, h:0, l:500000 (usec)           |      1.18% |
> |                            | fix_size_alloc_test: p:16, h:0, l:500000 (usec)          |     -5.01% |
> |                            | fix_size_alloc_test: p:16, h:1, l:500000 (usec)          |     13.81% |
> |                            | fix_size_alloc_test: p:64, h:0, l:100000 (usec)          |      6.51% |
> |                            | fix_size_alloc_test: p:64, h:1, l:100000 (usec)          |     32.87% |
> |                            | fix_size_alloc_test: p:256, h:0, l:100000 (usec)         |      4.17% |
> |                            | fix_size_alloc_test: p:256, h:1, l:100000 (usec)         |      8.40% |
> |                            | fix_size_alloc_test: p:512, h:0, l:100000 (usec)         |     -0.48% |
> |                            | fix_size_alloc_test: p:512, h:1, l:100000 (usec)         |     -0.74% |
> |                            | full_fit_alloc_test: p:1, h:0, l:500000 (usec)           |      0.53% |
> |                            | kvfree_rcu_1_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |     -2.81% |
> |                            | kvfree_rcu_2_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |     -2.06% |
> |                            | long_busy_list_alloc_test: p:1, h:0, l:500000 (usec)     |     -0.56% |
> |                            | pcpu_alloc_test: p:1, h:0, l:500000 (usec)               |     -0.41% |
> |                            | random_size_align_alloc_test: p:1, h:0, l:500000 (usec)  |      0.89% |
> |                            | random_size_alloc_test: p:1, h:0, l:500000 (usec)        |      1.71% |
> |                            | vm_map_ram_test: p:1, h:0, l:500000 (usec)               |      0.83% |
> +----------------------------+----------------------------------------------------------+------------+
> | schbench/thread-contention | -m 16 -t 1 -r 10 -s 1000, avg_rps (req/sec)              |      0.05% |
> |                            | -m 16 -t 1 -r 10 -s 1000, req_latency_p99 (usec)         |      0.60% |
> |                            | -m 16 -t 1 -r 10 -s 1000, wakeup_latency_p99 (usec)      |      0.00% |
> |                            | -m 16 -t 4 -r 10 -s 1000, avg_rps (req/sec)              |     -0.34% |
> |                            | -m 16 -t 4 -r 10 -s 1000, req_latency_p99 (usec)         |     -0.58% |
> |                            | -m 16 -t 4 -r 10 -s 1000, wakeup_latency_p99 (usec)      |      9.09% |
> |                            | -m 16 -t 16 -r 10 -s 1000, avg_rps (req/sec)             |     -0.74% |
> |                            | -m 16 -t 16 -r 10 -s 1000, req_latency_p99 (usec)        |     -1.40% |
> |                            | -m 16 -t 16 -r 10 -s 1000, wakeup_latency_p99 (usec)     |      0.00% |
> |                            | -m 16 -t 64 -r 10 -s 1000, avg_rps (req/sec)             |     -0.78% |
> |                            | -m 16 -t 64 -r 10 -s 1000, req_latency_p99 (usec)        |     -0.11% |
> |                            | -m 16 -t 64 -r 10 -s 1000, wakeup_latency_p99 (usec)     |      0.11% |
> |                            | -m 16 -t 256 -r 10 -s 1000, avg_rps (req/sec)            |      2.64% |
> |                            | -m 16 -t 256 -r 10 -s 1000, req_latency_p99 (usec)       |      3.15% |
> |                            | -m 16 -t 256 -r 10 -s 1000, wakeup_latency_p99 (usec)    |     17.54% |
> |                            | -m 32 -t 1 -r 10 -s 1000, avg_rps (req/sec)              |     -1.22% |
> |                            | -m 32 -t 1 -r 10 -s 1000, req_latency_p99 (usec)         |      0.85% |
> |                            | -m 32 -t 1 -r 10 -s 1000, wakeup_latency_p99 (usec)      |      0.00% |
> |                            | -m 32 -t 4 -r 10 -s 1000, avg_rps (req/sec)              |     -0.34% |
> |                            | -m 32 -t 4 -r 10 -s 1000, req_latency_p99 (usec)         |      1.05% |
> |                            | -m 32 -t 4 -r 10 -s 1000, wakeup_latency_p99 (usec)      |      0.00% |
> |                            | -m 32 -t 16 -r 10 -s 1000, avg_rps (req/sec)             |     -0.41% |
> |                            | -m 32 -t 16 -r 10 -s 1000, req_latency_p99 (usec)        |      0.58% |
> |                            | -m 32 -t 16 -r 10 -s 1000, wakeup_latency_p99 (usec)     |      2.13% |
> |                            | -m 32 -t 64 -r 10 -s 1000, avg_rps (req/sec)             |      0.67% |
> |                            | -m 32 -t 64 -r 10 -s 1000, req_latency_p99 (usec)        |      2.07% |
> |                            | -m 32 -t 64 -r 10 -s 1000, wakeup_latency_p99 (usec)     |     -1.28% |
> |                            | -m 32 -t 256 -r 10 -s 1000, avg_rps (req/sec)            |      1.01% |
> |                            | -m 32 -t 256 -r 10 -s 1000, req_latency_p99 (usec)       |      0.69% |
> |                            | -m 32 -t 256 -r 10 -s 1000, wakeup_latency_p99 (usec)    |     13.12% |
> |                            | -m 64 -t 1 -r 10 -s 1000, avg_rps (req/sec)              |     -0.25% |
> |                            | -m 64 -t 1 -r 10 -s 1000, req_latency_p99 (usec)         |     -0.48% |
> |                            | -m 64 -t 1 -r 10 -s 1000, wakeup_latency_p99 (usec)      |     10.53% |
> |                            | -m 64 -t 4 -r 10 -s 1000, avg_rps (req/sec)              |     -0.06% |
> |                            | -m 64 -t 4 -r 10 -s 1000, req_latency_p99 (usec)         |      0.00% |
> |                            | -m 64 -t 4 -r 10 -s 1000, wakeup_latency_p99 (usec)      |      0.00% |
> |                            | -m 64 -t 16 -r 10 -s 1000, avg_rps (req/sec)             |     -0.36% |
> |                            | -m 64 -t 16 -r 10 -s 1000, req_latency_p99 (usec)        |      0.52% |
> |                            | -m 64 -t 16 -r 10 -s 1000, wakeup_latency_p99 (usec)     |      0.11% |
> |                            | -m 64 -t 64 -r 10 -s 1000, avg_rps (req/sec)             |      0.52% |
> |                            | -m 64 -t 64 -r 10 -s 1000, req_latency_p99 (usec)        |      3.53% |
> |                            | -m 64 -t 64 -r 10 -s 1000, wakeup_latency_p99 (usec)     |     -0.10% |
> |                            | -m 64 -t 256 -r 10 -s 1000, avg_rps (req/sec)            |      2.53% |
> |                            | -m 64 -t 256 -r 10 -s 1000, req_latency_p99 (usec)       |      1.82% |
> |                            | -m 64 -t 256 -r 10 -s 1000, wakeup_latency_p99 (usec)    |     -5.80% |
> +----------------------------+----------------------------------------------------------+------------+
> | syscall/getpid             | mean (ns)                                                | (I) 15.98% |
> |                            | p99 (ns)                                                 | (I) 11.11% |
> |                            | p99.9 (ns)                                               | (I) 16.13% |
> +----------------------------+----------------------------------------------------------+------------+
> | syscall/getppid            | mean (ns)                                                | (I) 14.82% |
> |                            | p99 (ns)                                                 | (I) 17.86% |
> |                            | p99.9 (ns)                                               |  (I) 9.09% |
> +----------------------------+----------------------------------------------------------+------------+
> | syscall/invalid            | mean (ns)                                                | (I) 17.78% |
> |                            | p99 (ns)                                                 | (I) 11.11% |
> |                            | p99.9 (ns)                                               |     13.33% |
> +----------------------------+----------------------------------------------------------+------------+
> 
> [1] https://gitlab.arm.com/tooling/fastpath
> 
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
> ---
> The patch applies on v6.19 and next-20260309.
> ---
>  arch/arm64/include/asm/mte.h | 6 +++++-
>  arch/arm64/kernel/mte.c      | 5 +++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index 6d4a78b9dc3e6..0e05d20cf2583 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -252,7 +252,8 @@ static inline void mte_check_tfsr_entry(void)
>  	if (!kasan_hw_tags_enabled())
>  		return;
>  
> -	mte_check_tfsr_el1();
> +	if (system_uses_mte_async_or_asymm_mode())
> +		mte_check_tfsr_el1();
>  }
>  
>  static inline void mte_check_tfsr_exit(void)
> @@ -260,6 +261,9 @@ static inline void mte_check_tfsr_exit(void)
>  	if (!kasan_hw_tags_enabled())
>  		return;
>  
> +	if (!system_uses_mte_async_or_asymm_mode())
> +		return;
> +
>  	/*
>  	 * The asynchronous faults are sync'ed automatically with
>  	 * TFSR_EL1 on kernel entry but for exit an explicit dsb()
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 32148bf09c1dc..8da2891b834d7 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -291,6 +291,8 @@ void mte_thread_switch(struct task_struct *next)
>  	/* TCO may not have been disabled on exception entry for the current task. */
>  	mte_disable_tco_entry(next);
>  
> +	if (!system_uses_mte_async_or_asymm_mode())
> +		return;
>  	/*
>  	 * Check if an async tag exception occurred at EL1.
>  	 *
> @@ -350,6 +352,9 @@ void mte_suspend_enter(void)
>  	if (!system_supports_mte())
>  		return;
>  
> +	if (!system_uses_mte_async_or_asymm_mode())
> +		return;
> +
>  	/*
>  	 * The barriers are required to guarantee that the indirect writes
>  	 * to TFSR_EL1 are synchronized before we report the state.