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

Marcelo Tosatti
2012-06-12T22:27:48Z

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: