Re: [PATCH 12/13] arm64: Add a capability for FEAT_EVC

[Marc Zyngier <maz@kernel.org>]

Hi Oliver,

Thanks for having a look.

On Mon, 09 Aug 2021 17:30:45 +0100,
Oliver Upton <oupton@google.com> wrote:
> 
> Hi Marc,
> 
> On Mon, Aug 9, 2021 at 8:48 AM Marc Zyngier <maz@kernel.org> wrote:
> >
> > Add a new capability to detect the Enhanced Counter Virtualization
> > feature (FEAT_EVC).
> >
> 
> s/FEAT_EVC/FEAT_ECV/g

I'm the knig fo tpyoes :).

> 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kernel/cpufeature.c | 10 ++++++++++
> >  arch/arm64/tools/cpucaps       |  1 +
> >  2 files changed, 11 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 0ead8bfedf20..9c2ce5408811 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1899,6 +1899,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
> >                 .sign = FTR_UNSIGNED,
> >                 .min_field_value = 1,
> >         },
> > +       {
> > +               .desc = "Enhanced counter virtualization",
> > +               .capability = ARM64_HAS_ECV,
> > +               .type = ARM64_CPUCAP_SYSTEM_FEATURE,
> > +               .matches = has_cpuid_feature,
> > +               .sys_reg = SYS_ID_AA64MMFR0_EL1,
> > +               .field_pos = ID_AA64MMFR0_ECV_SHIFT,
> > +               .sign = FTR_UNSIGNED,
> > +               .min_field_value = 1,
> > +       },
> 
> Per one of your other patches in the series, it sounds like userspace
> access to the self-synchronized registers hasn't been settled yet.
> However, if/when available to userspace, should this cpufeature map to
> an ELF HWCAP?

We can't prevent the access to userspace, unless we also trap
cntvct_el0 and cntfreq_el0. Which we try not to do. But you are indeed
correct, we probably have a HWCAP if we decide to advertise it to
userspace.

> Also, w.r.t. my series I have out for ECV in KVM. All the controls
> used in EL2 depend on ECV=0x2. I agree that ECV=0x1 needs a cpufeature
> bit, but what about EL2's use case?

My idea was to have a ARM64_HAS_ECV2 to capture the EL2 extensions
with min_field_value=2.

> Besides the typo:
> 
> Reviewed-by: Oliver Upton <oupton@google.com>

Thanks,

	M.

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