From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [v6] kvm/fpu: Enable fully eager restore kvm FPU Date: Fri, 24 Apr 2015 09:44:29 +0200 Message-ID: <5539F45D.6020400@redhat.com> References: <1429823583-3226-1-git-send-email-liang.z.li@intel.com> <55390F9A.2070808@intel.com> <553955D9.3030600@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: "gleb@kernel.org" , "mtosatti@redhat.com" , "tglx@linutronix.de" , "mingo@redhat.com" , "x86@kernel.org" , "joro@8bytes.org" , "Hao, Xudong" To: "Zhang, Yang Z" , "H. Peter Anvin" , "Hansen, Dave" , "Li, Liang Z" , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 24/04/2015 03:16, Zhang, Yang Z wrote: >> This is interesting since previous measurements on KVM have had >> the exact opposite results. I think we need to understand this a >> lot more. > > What I can tell is that vmexit is heavy. So it is reasonable to see > the improvement under some cases, especially kernel is using eager > FPU now which means each schedule may trigger a vmexit. On the other hand vmexit is lighter and lighter on newer processors; a Sandy Bridge has less than half the vmexit cost of a Core 2 (IIRC 1000 vs. 2500 clock cycles approximately). Also, measurement were done on Westmere but Sandy Bridge is the first processor to have XSAVEOPT and thus use eager FPU. Paolo