From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [RFC PATCH] Emulate MOVBE Date: Wed, 10 Apr 2013 13:08:46 +0300 Message-ID: <20130410100845.GB17919@redhat.com> References: <20130409234602.GI5077@pd.tnic> <20130410112942.07dfc167@slackpad> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Borislav Petkov , kvm@vger.kernel.org, =?utf-8?B?SsO2cmcgUsO2ZGVs?= , "H. Peter Anvin" , x86-ml , kvm@vger-kernel.org To: Andre Przywara Return-path: Received: from mx1.redhat.com ([209.132.183.28]:9899 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751202Ab3DJKJH (ORCPT ); Wed, 10 Apr 2013 06:09:07 -0400 Content-Disposition: inline In-Reply-To: <20130410112942.07dfc167@slackpad> Sender: kvm-owner@vger.kernel.org List-ID: On Wed, Apr 10, 2013 at 11:29:42AM +0200, Andre Przywara wrote: > In a real world VendorSpecific should be replaced with something more > meaningful. Depends on KVMs intention to emulate instructions, actually > out of scope for a pure virtualizer. > Something like EmulateOnUD. > 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. > > (And if one can believe the AMD Fam16h SWOG [1], PS4^Wfuture AMD > processors have MOVBE, so it's not even actually one CPU anymore). If a host CPU has the instruction emulation is not needed unless the instruction is used for MMIO access. -- Gleb.