Re: [PATCHv6 6/8] kvm: only sync when attention bits set

Michael S. Tsirkin
2012-06-13T08:19:24Z

On Tue, Jun 12, 2012 at 07:27:48PM -0300, Marcelo Tosatti wrote:
> On Sun, Jun 03, 2012 at 10:28:29AM +0300, Michael S. Tsirkin wrote:
> > Commit eb0dc6d0368072236dcd086d7fdc17fd3c4574d4 introduced apic
> > attention bitmask but kvm still syncs lapic unconditionally.
> > As that commit suggested and in anticipation of adding more attention
> > bits, only sync lapic if(apic_attention).
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > arch/x86/kvm/x86.c | 3 ++-
> > 1 files changed, 2 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index be6d549..2f70861 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -5388,7 +5388,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > if (unlikely(vcpu->arch.tsc_always_catchup))
> > kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> >
> > - kvm_lapic_sync_from_vapic(vcpu);
> > + if (unlikely(vcpu->arch.apic_attention))
> > + kvm_lapic_sync_from_vapic(vcpu);
>
> void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu)
> {
> u32 data;
> void *vapic;
>
> if (!test_bit(KVM_APIC_CHECK_VAPIC, &vcpu->arch.apic_attention))
> return;
>
> Please use unlikely more carefully, when a gain is measureable:
> http://lwn.net/Articles/420019/

Do we have to measure every single thing?
Sometimes it's obvious: vapic is slow path, isn't it?

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MST