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

[Fuad Tabba]

On Mon, 24 Aug 2026 at 21:29, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> 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.

Like this?

      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));
      }

> 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).

Agreed it's separate, but v3 applies the override without checking
that the CPU has MTE2, so id_aa64pfr1.mte=2 on a CPU without it reads
a GMID_EL1 that isn't there. On the boot CPU that's before the
override is sanitised. Should I check the raw register before applying
the override, until the clamp lands?

>
> 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().

Not worried :) __cpuinfo_store_cpu() has already read ID_AA64PFR1_EL1
a few lines up.

Cheers,
/fuad


>
> --
> Catalin