From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [RFC PATCH] Emulate MOVBE Date: Tue, 16 Apr 2013 20:28:56 +0300 Message-ID: <20130416172856.GD5807@redhat.com> References: <20130409234602.GI5077@pd.tnic> <20130410112942.07dfc167@slackpad> <20130410100845.GB17919@redhat.com> <516D3A62.6030708@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andre Przywara , Borislav Petkov , kvm@vger.kernel.org, =?utf-8?B?SsO2cmcgUsO2ZGVs?= , "H. Peter Anvin" , x86-ml , kvm@vger-kernel.org To: Paolo Bonzini Return-path: Received: from mx1.redhat.com ([209.132.183.28]:3646 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936020Ab3DPR3M (ORCPT ); Tue, 16 Apr 2013 13:29:12 -0400 Content-Disposition: inline In-Reply-To: <516D3A62.6030708@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Apr 16, 2013 at 01:47:46PM +0200, Paolo Bonzini wrote: > Il 10/04/2013 12:08, Gleb Natapov ha scritto: > >> > What is the opinion from the KVM folks on this? Shall we start to > >> > emulate instructions the host does not provide? In this particular case > >> > a relatively simple patch fixes a problem (starting Atom optimized > >> > kernels on non-Atom machines). > > We can add the emulation, but we should not start announcing the instruction > > availability to a guest if host cpu does not have it by default. This > > may trick a guest into thinking that movbe is the fastest way to do > > something when it is not. > > > > This does highlight a weakness in CPU_GET_SUPPORTED_CPUID, but I think > this is not a problem in practice. > > With a management layer such as oVirt it's not a problem. For example, > oVirt has its own library of processors. It doesn't care if KVM enables > movbe. If you tell it your datacenter is a mix of Haswells and Sandy > Bridges it will pick the CPUID bits that are common to all. > > However, even without a suitable management layer it is also not really > a problem. > > The only processors that support MOVBE are Atom and Haswell. Haswell > adds a whole lot of extra CPUID features, hence "-cpu Haswell,enforce" > will fail with or without movbe emulation. "-cpu Haswell" will disable > all Haswell new features except movbe will remain slow; that's fine, I > think, anyway it's not what you'ld do except to play with CPU models. > No that's not fine. KVM should not trick userspace (QEMU is just one of them) into nonoptimal configuration. And you forgot about -cpu host in your analysis. -- Gleb.