Re: [PATCH v4 2/2] arm64: Don't read GMID_EL1 when MTE is disabled

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

On Thu, Aug 27, 2026 at 01:56:35PM +0100, Will Deacon wrote:
> On Tue, Aug 25, 2026 at 05:42:19PM +0100, Fuad Tabba wrote:
> > +/* No ID register reflects CONFIG_ARM64_MTE. */
> > +static inline bool gmid_el1_accessible(void)
> > +{
> > +	if (!IS_ENABLED(CONFIG_ARM64_MTE))
> > +		return false;
> > +
> > +	return id_aa64pfr1_mte(__read_sysreg_by_encoding(SYS_ID_AA64PFR1_EL1));
> 
> I'm planning to internalise __read_sysreg_by_encoding() into cpufeature.c
> so I'd prefer to avoid adding another user of it, if possible. In this case,
> __cpuinfo_store_cpu() has already read the thing, so all it needs is the
> override logic (but see below).

That's a valid point.

> > @@ -502,7 +502,7 @@ static void __cpuinfo_store_cpu(struct cpuinfo_arm64 *info)
> >  	info->reg_id_aa64smfr0 = read_cpuid(ID_AA64SMFR0_EL1);
> >  	info->reg_id_aa64fpfr0 = read_cpuid(ID_AA64FPFR0_EL1);
> >  
> > -	if (id_aa64pfr1_mte(info->reg_id_aa64pfr1))
> > +	if (gmid_el1_accessible())
> >  		info->reg_gmid = read_cpuid(GMID_EL1);
> 
> I'm not sure this is safe. For the boot CPU, __cpuinfo_store_cpu() is
> called before init_cpu_features(), so the arm64_ftr_regs[] array hasn't
> been sorted and we can't call get_arm64_ftr_reg() reliably.

We no longer sort this array at boot-time, we just check that it's
pre-sorted.

> In fact, it
> doesn't even look like the overrides will have been processed.

init_feature_override() runs from early_map_kernel() but yes,
init_cpu_ftr_reg() hasn't been called yet to vet the override. Suzuki's
patch was adding extra checks in __read_sysreg_by_encoding() but if we
drop that patch (and function eventually), better not to rely on it.

So I don't think we have a correctness issue here but I don't mind v5
either. I'll go and check.

-- 
Catalin