[PATCH 1/9] KVM: arm64: selftests: Add helpers to extract a field of an ID register

[Reiji Watanabe]

Hi Oliver, Ricardo,

On Thu, Aug 25, 2022 at 9:58 AM Oliver Upton <oliver.upton at linux.dev> wrote:
>
> On Thu, Aug 25, 2022 at 09:52:53AM -0700, Ricardo Koller wrote:
> > On Thu, Aug 25, 2022 at 09:48:35AM -0700, Oliver Upton wrote:
> > > Hi Reiji,
> > >
> > > On Wed, Aug 24, 2022 at 10:08:38PM -0700, Reiji Watanabe wrote:
> > > > Introduce helpers to extract a field of an ID register.
> > > > Subsequent patches will use those helpers.
> > > >
> > > > Signed-off-by: Reiji Watanabe <reijiw at google.com>
> > > > ---
> > > >  .../selftests/kvm/include/aarch64/processor.h     |  2 ++
> > > >  .../testing/selftests/kvm/lib/aarch64/processor.c | 15 +++++++++++++++
> > > >  2 files changed, 17 insertions(+)
> > > >
> > > > diff --git a/tools/testing/selftests/kvm/include/aarch64/processor.h b/tools/testing/selftests/kvm/include/aarch64/processor.h
> > > > index a8124f9dd68a..a9b4b4e0e592 100644
> > > > --- a/tools/testing/selftests/kvm/include/aarch64/processor.h
> > > > +++ b/tools/testing/selftests/kvm/include/aarch64/processor.h
> > > > @@ -193,4 +193,6 @@ void smccc_hvc(uint32_t function_id, uint64_t arg0, uint64_t arg1,
> > > >
> > > >  uint32_t guest_get_vcpuid(void);
> > > >
> > > > +int cpuid_get_sfield(uint64_t val, int field_shift);
> > > > +unsigned int cpuid_get_ufield(uint64_t val, int field_shift);
> > > >  #endif /* SELFTEST_KVM_PROCESSOR_H */
> > > > diff --git a/tools/testing/selftests/kvm/lib/aarch64/processor.c b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> > > > index 6f5551368944..0b2ad46e7ff5 100644
> > > > --- a/tools/testing/selftests/kvm/lib/aarch64/processor.c
> > > > +++ b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> > > > @@ -528,3 +528,18 @@ void smccc_hvc(uint32_t function_id, uint64_t arg0, uint64_t arg1,
> > > >                  [arg4] "r"(arg4), [arg5] "r"(arg5), [arg6] "r"(arg6)
> > > >                : "x0", "x1", "x2", "x3", "x4", "x5", "x6", "x7");
> > > >  }
> > > > +
> > > > +/* Helpers to get a signed/unsigned feature field from ID register value */
> > > > +int cpuid_get_sfield(uint64_t val, int field_shift)
> > > > +{
> > > > + int width = 4;
> > > > +
> > > > + return (int64_t)(val << (64 - width - field_shift)) >> (64 - width);
> > > > +}
> > >
> > > I don't believe this helper is ever used.

I thought I was going to use this in the selftest for my ID register series.
(So, I originally would like to have similar helpers in cpufeture.h)
But, you are right. That test only extracts unsigned fields...

> > >
> > > > +unsigned int cpuid_get_ufield(uint64_t val, int field_shift)
> > > > +{
> > > > + int width = 4;
> > > > +
> > > > + return (uint64_t)(val << (64 - width - field_shift)) >> (64 - width);
> > > > +}
> > >
> > > I would recommend not open-coding this and instead make use of
> > > ARM64_FEATURE_MASK(). You could pull in linux/bitfield.h to tools, or do
> > > something like this:
> > >
> > >   #define ARM64_FEATURE_GET(ftr, val)                                       \
> > >               ((ARM64_FEATURE_MASK(ftr) & val) >> ftr##_SHIFT)
> > >
> > > Slight preference for FIELD_{GET,SET}() as it matches the field
> > > extraction in the kernel as well.
> >
> > Was doing that with this commit:
> >
> >       [PATCH v5 05/13] tools: Copy bitfield.h from the kernel sources
> >
> > Maybe you could just use it given that it's already reviewed.
>
> Oops, thanks for the reminder Ricardo! Yeah, let's go that route then.

Thank you for the information. I will use that instead.

Thank you,
Reiji