[PATCH v3] arm64: Don't read GMID_EL1 when MTE is disabled

[Catalin Marinas]

On Mon, Aug 24, 2026 at 07:41:55PM +0100, Fuad Tabba wrote:
> +/*
> + * Reading GMID_EL1 when the kernel has disabled MTE traps to EL2, and the raw
> + * ID_AA64PFR1_EL1 reflects neither CONFIG_ARM64_MTE nor the cmdline override.
> + */
> +static inline bool gmid_el1_accessible(u64 pfr1)
> +{
> +	if (!IS_ENABLED(CONFIG_ARM64_MTE))
> +		return false;
> +
> +	pfr1 &= ~id_aa64pfr1_override.mask;
> +	pfr1 |= id_aa64pfr1_override.val;
> +
> +	return id_aa64pfr1_mte(pfr1);
> +}

Can this function not use __read_sysreg_by_encoding() directly and not
get an argument at all? We'd get the override from that function rather
than open-coding it here. Eventually, once we get Suzuki's clamping fix,
it also ensures we won't be able to bump the MTE version to unsafe
values (but that's a different fix from this one).

If you are worried about additional trapping of the MRS, we should go
for Will's improvement to always read the cached values in
__read_sysreg_by_encoding().

-- 
Catalin