Re: [PATCH v2] arm64: Defer the GMID_EL1 read to {init,update}_cpu_features()

[Fuad Tabba]

Hi Will,

On Mon, 24 Aug 2026 at 14:09, Will Deacon <will@xxxxxxxxxx> wrote:

> > +     /*
> > +      * info->reg_gmid deferred to {init,update}_cpu_features because
> > +      * reading it traps to EL2 when MTE is disabled.
> > +      */
>
> I don't think we should defer this, as I've been actively doing the
> opposite for parallel CPU onlining (where the ID registers can be read
> concurrently by incoming CPUs to amortise the cost of a trap) and also
> for the RNG traps during early boot:
>
> https://lore.kernel.org/all/annJ0oDB2HObQC5j@willie-the-truck/
>
> If you look at the diff I sent in the thread above (I didn't get a
> reply), the idea is that __read_sysreg_by_encoding() reads from the ID
> register values stashed by cpuinfo_store_cpu(). So you could use that
> to check id_aa64pfr1 before reading gmid, as it will give you the
> sanitised view.

I just did, but I don't think it'll work. The thing is,
__read_sysreg_by_encoding() gives the override-applied local value,
not the folded sys_val.

arm64.nomte is a command-line override, so that works. But
CONFIG_ARM64_MTE=n sets none. Any gate would still need an explicit
IS_ENABLED(CONFIG_ARM64_MTE).

That said, I don't have to defer it. I can keep the info->reg_gmid
read in __cpuinfo_store_cpu() and gate that read on the state that
arms the trap:

        static inline bool gmid_el1_accessible(u64 pfr1)
        {
                if (!IS_ENABLED(CONFIG_ARM64_MTE))
                        return false;
                if (system_capabilities_finalized())
                        return system_supports_mte();
                return id_aa64pfr1_mte(pfr1);
        }


TID5 is set from system_supports_mte() and only once capabilities are
finalised, so the local check is still correct before that. Gating on
system_supports_mte() keeps the different-physical-CPU case you want
to preserve working: if one re-onlines with MTE while the system view
has it folded off, the gate stays false and it won't read GMID_EL1.

If you agree, I'll respin with that.

Cheers,
/fuad





>
> The only snag is that I didn't convert all the registers over, so
> there's some work there.
>
> WDYT?
>
> Will