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

[Marcelo Tosatti <mtosatti@redhat.com>]

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/