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

From: Oliver Upton <oliver.upton@linux.dev>
To: Ricardo Koller <ricarkol@google.com>
Cc: Reiji Watanabe <reijiw@google.com>,
	kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	Andrew Jones <andrew.jones@linux.dev>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/9] KVM: arm64: selftests: Add helpers to extract a field of an ID register
Date: Thu, 25 Aug 2022 09:58:09 -0700	[thread overview]
Message-ID: <YweqIefFbP107fe+@google.com> (raw)
In-Reply-To: <Yweo5cmA6D0pxwmJ@google.com>

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@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.
> > 
> > > +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.

--
Thanks,
Oliver

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