From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755045Ab2CLMNI (ORCPT ); Mon, 12 Mar 2012 08:13:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:64041 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754796Ab2CLMNG (ORCPT ); Mon, 12 Mar 2012 08:13:06 -0400 Message-ID: <4F5DE84B.8050606@redhat.com> Date: Mon, 12 Mar 2012 14:12:59 +0200 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120216 Thunderbird/10.0.1 MIME-Version: 1.0 To: Christian Borntraeger CC: Davidlohr Bueso , Marcelo Tosatti , KVM , lkml Subject: Re: [PATCH v2] KVM: x86: add paging gcc optimization References: <1331207154.28711.2.camel@offworld> <4F58D48F.8050807@de.ibm.com> In-Reply-To: <4F58D48F.8050807@de.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/08/2012 05:47 PM, 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. Agree. > Is that the case here? This patch might cause a mis-prediction for non-paging guests all > the time. > > 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". In fact this is a classic example. Almost no guests use real mode (the last guests to use real mode extensively was DOS; I think Win9x switches to real mode pretty often). As it's a user-controlled setting, we're penalizing users who do things differently. However the majority if is_paging() == true guests is so huge, and since non-paging guests don't really expect 2012 performance levels anyway (being so old) that I think in practice this is a good optimization here. -- error compiling committee.c: too many arguments to function