Re: [PATCH v2 07/14] KVM: arm64: Use a cpucap to determine if system supports FEAT_PMUv3

From: Marc Zyngier
Date: Wed Feb 19 2025 - 14:37:47 EST


On Wed, 19 Feb 2025 19:22:56 +0000,
Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> On Wed, Feb 19, 2025 at 05:44:59PM +0000, Marc Zyngier wrote:
> > > +static bool has_pmuv3(const struct arm64_cpu_capabilities *entry, int scope)
> > > +{
> > > + u64 dfr0 = read_sanitised_ftr_reg(SYS_ID_AA64DFR0_EL1);
> > > + unsigned int pmuver;
> > > +
> > > + pmuver = cpuid_feature_extract_unsigned_field(dfr0,
> > > + ID_AA64DFR0_EL1_PMUVer_SHIFT);
> > > + if (pmuver == ID_AA64DFR0_EL1_PMUVer_IMP_DEF)
> > > + return false;
> > > +
> > > + return pmuver >= ID_AA64DFR0_EL1_PMUVer_IMP;
> >
> > Given that PMUVer is a signed field, how about using
> > cpuid_feature_extract_signed_field() and do a signed comparison instead?
>
> I'm happy to include a comment, but the PMUVer field is not signed. Any value
> other than 0xF is meant to be treated as an unsigned quantity.
>
> DDI047L.a D24.1.3.2 is where this is coming from.

Duh, you're of course correct. Ignore me.

M.

--
Without deviation from the norm, progress is not possible.