Re: [PATCH v2 2/2] arm64: mte: Set TCMA1 whenever MTE is present in the kernel

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jan 15, 2026 at 03:07:18PM -0800, Carl Worth wrote:
> 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 <taehyun@utexas.edu>
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Carl Worth <carl@os.amperecomputing.com>

Thanks for testing an sending this.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>