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

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Message-ID: <20120612222748.GB1973@amt.cnet>
Date:	Tue, 12 Jun 2012 19:27:48 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	x86@...nel.org, kvm@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	gleb@...hat.com, Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv6 6/8] kvm: only sync when attention bits set

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@...hat.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/

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