On Thu, Apr 04, 2019 at 04:17:24AM +0100, Marc Zyngier wrote:
> On Wed, 03 Apr 2019 20:14:13 +0100,
> Andrew Jones <drjones@xxxxxxxxxx> wrote:
> >
> > On Fri, Mar 29, 2019 at 01:00:34PM +0000, Dave Martin wrote:
> > > Since SVE will be enabled or disabled on a per-vcpu basis, a flag
> > > is needed in order to track which vcpus have it enabled.
> > >
> > > This patch adds a suitable flag and a helper for checking it.
> > >
> > > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> > > Reviewed-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
> > > Tested-by: zhang.lei <zhang.lei@xxxxxxxxxxxxxx>
> > > ---
> > > arch/arm64/include/asm/kvm_host.h | 4 ++++
> > > 1 file changed, 4 insertions(+)
> > >
> > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > index 6d10100..ad4f7f0 100644
> > > --- a/arch/arm64/include/asm/kvm_host.h
> > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > @@ -328,6 +328,10 @@ struct kvm_vcpu_arch {
> > > #define KVM_ARM64_FP_HOST (1 << 2) /* host FP regs loaded */
> > > #define KVM_ARM64_HOST_SVE_IN_USE (1 << 3) /* backup for host TIF_SVE */
> > > #define KVM_ARM64_HOST_SVE_ENABLED (1 << 4) /* SVE enabled for EL0 */
> > > +#define KVM_ARM64_GUEST_HAS_SVE (1 << 5) /* SVE exposed to guest */
> > > +
> > > +#define vcpu_has_sve(vcpu) (system_supports_sve() && \
> > > + ((vcpu)->arch.flags & KVM_ARM64_GUEST_HAS_SVE))
> >
> > We shouldn't need the system_supports_sve() here. vcpu->arch.flags can
> > only have the KVM_ARM64_GUEST_HAS_SVE flag set when system_supports_sve()
> > is true, and it can't change later.
>
> This is a performance optimisation. system_supports_sve() results in a
> static key being emitted, and that avoids a number of loads on the
> fast path for non-SVE systems (which is likely to be the absolute
> majority for the foreseeable future).
Exactly so. The same applies to many of the other call sites for
system_supports_sve().
Cheers
---Dave
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