From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751451Ab2FMIet (ORCPT ); Wed, 13 Jun 2012 04:34:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:31242 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750727Ab2FMIeq (ORCPT ); Wed, 13 Jun 2012 04:34:46 -0400 Date: Wed, 13 Jun 2012 11:35:07 +0300 From: "Michael S. Tsirkin" To: Marcelo Tosatti Cc: x86@kernel.org, kvm@vger.kernel.org, Ingo Molnar , "H. Peter Anvin" , Avi Kivity , gleb@redhat.com, Linus Torvalds , linux-kernel@vger.kernel.org, Thomas Gleixner Subject: Re: [PATCHv6 6/8] kvm: only sync when attention bits set Message-ID: <20120613083506.GA17190@redhat.com> References: <56ee14064ef307b7075f6977122781456cab5165.1338474301.git.mst@redhat.com> <20120612222748.GB1973@amt.cnet> <20120613081923.GA16162@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120613081923.GA16162@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > --- > > > 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