Re: [PATCH v7 09/27] KVM: arm64: Add a vcpu flag to control SVE visibility for the guest

Marc Zyngier
2019-04-04T03:17:40Z

On Wed, 03 Apr 2019 20:14:13 +0100,
Andrew Jones <drjones@redhat.com> 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@arm.com>
> > Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> > Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> > ---
> > 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).

Thanks,

M.

--
Jazz is not dead, it just smell funny.