Re: [PATCH RFC v2 04/10] target/arm: add FEAT_MTE_STORE_ONLY logic

[Richard Henderson <richard.henderson@linaro.org>]

On 11/17/25 02:40, Gabriel Brookman wrote:
> This feature automatically succeeds tag checks on load instructions when
> the appropriate SCTLR_TCSO register for the current exception level is
> set. Find information on this feature in the "Tag Checking" section of
> the ARM ARM, Memory Tagging Extension chapter.
> 
> Signed-off-by: Gabriel Brookman <brookmangabriel@gmail.com>
> ---
>   target/arm/tcg/mte_helper.c | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/target/arm/tcg/mte_helper.c b/target/arm/tcg/mte_helper.c
> index bb48fe359b..f9fd6fd408 100644
> --- a/target/arm/tcg/mte_helper.c
> +++ b/target/arm/tcg/mte_helper.c
> @@ -865,8 +865,30 @@ static int mte_probe_int(CPUARMState *env, uint32_t desc, uint64_t ptr,
>       return 0;
>   }
>   
> +static bool mte_store_only_active(CPUARMState *env)
> +{
> +    int el = arm_current_el(env);
> +    if (el) {
> +        if (SCTLR_TCSO & env->cp15.sctlr_el[el]) {
> +            return true;
> +        }
> +    } else {
> +        if ((HCR_E2H & env->cp15.hcr_el2) &&'

You can't test hcr_el2 directly, because SecEL2 might not exist when (Non-secure) EL2 
does.  However, this can be simplified using arm_sctlr():

     uint64_t sctlr = arm_sctlr(env, el);
     uint64_t test = el ? SCTLR_TCSO : SCTLR_TCSO0;
     return sctlr & test;


That said, we should cache this in TBFLAGS so that we don't generate the mte_check 
function call at runtime for reads.  See target/arm/tcg/hflags.c, s/MTE_ACTIVE/.


r~