From mboxrd@z Thu Jan 1 00:00:00 1970 From: Davidlohr Bueso Subject: Re: [PATCH v2] KVM: x86: add paging gcc optimization Date: Thu, 08 Mar 2012 22:27:59 +0100 Message-ID: <1331242079.2592.4.camel@offbook> References: <1331207154.28711.2.camel@offworld> <4F58D48F.8050807@de.ibm.com> Reply-To: dave@gnu.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Avi Kivity , Marcelo Tosatti , KVM , lkml To: Christian Borntraeger Return-path: Received: from caiajhbdccac.dreamhost.com ([208.97.132.202]:58577 "EHLO homiemail-a60.g.dreamhost.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752924Ab2CHV2F (ORCPT ); Thu, 8 Mar 2012 16:28:05 -0500 In-Reply-To: <4F58D48F.8050807@de.ibm.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, 2012-03-08 at 16:47 +0100, Christian Borntraeger wrote: > On 08/03/12 12:45, Davidlohr Bueso wrote: > > From: Davidlohr Bueso > > > > Since most guests will have paging enabled for memory management, add likely() optimization > > around CR0.PG checks. > > > { > > - return kvm_read_cr0_bits(vcpu, X86_CR0_PG); > > + return likely(kvm_read_cr0_bits(vcpu, X86_CR0_PG)); > > > IMHO likely/unlikely should be considered more as fast-path/slow-path and not as often/less often. > Is that the case here? This patch might cause a mis-prediction for non-paging guests all > the time. Branch predictions are all about probability of occurrence, and I cannot imagine guests having paging disabled and mapping gva->gpa directly, so for most of the cases it would be beneficial. For those peculiar users that actually don't use paging, then yes, mispredictions would occur. - Davidlohr > > Non-paging might be really irrelevant, so I am just making a point, since > likely/unlikely is mis-used too often especially for "most users do it that way". > > > Christian > >