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

Michael S. Tsirkin
2012-06-13T08:35:07Z

On Wed, Jun 13, 2012 at 11:19:24AM +0300, Michael S. Tsirkin wrote:
> 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?

Just to clarify the question: I think it's obvious this condition is
false more often than true. By how much, depends on the workload.
Do you think this is enough to tag this unlikely?

> --
> MST