From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH v2] KVM: x86: add paging gcc optimization Date: Mon, 12 Mar 2012 14:12:59 +0200 Message-ID: <4F5DE84B.8050606@redhat.com> References: <1331207154.28711.2.camel@offworld> <4F58D48F.8050807@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Davidlohr Bueso , Marcelo Tosatti , KVM , lkml To: Christian Borntraeger Return-path: In-Reply-To: <4F58D48F.8050807@de.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.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