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

[Fuad Tabba]

On Tue, 25 Aug 2026 at 15:14, Will Deacon <will@xxxxxxxxxx> wrote:
...

> > 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?
>
> At which point, why don't we bite the bullet and implement Suzuki's
> idea so that __read_sysreg_by_encoding() does what you want more generally?
>
> https://lore.kernel.org/all/afc5bd00-28ca-413b-b047-ee53589c285d@xxxxxxx/

v4 will be two patches: Suzuki's clamp first, then the GMID_EL1 gate
on __read_sysreg_by_encoding().

>
> > > 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.
>
> But if you go with __read_sysreg_by_encoding() then you'd still be
> reading it (and trapping) twice, no?

It would, and I'm fine with that on a CPU bring-up path.

Stay tuned!
/fuad
>
> Will