From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andre Przywara Subject: Re: [RFC PATCH] Emulate MOVBE Date: Wed, 10 Apr 2013 12:39:01 +0200 Message-ID: <20130410123901.46b65169@slackpad> References: <20130409234602.GI5077@pd.tnic> <20130410112942.07dfc167@slackpad> <20130410100845.GB17919@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Borislav Petkov , kvm@vger.kernel.org, =?ISO-8859-1?Q?J?= =?ISO-8859-1?Q?=F6rg_R=F6del?= , "H. Peter Anvin" , x86-ml To: Gleb Natapov Return-path: Received: from mail.andrep.de ([217.160.17.100]:36504 "EHLO mail.andrep.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751549Ab3DJKjW (ORCPT ); Wed, 10 Apr 2013 06:39:22 -0400 In-Reply-To: <20130410100845.GB17919@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Wed, 10 Apr 2013 13:08:46 +0300 Gleb Natapov wrote: > 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. Right. > > 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. Good point. I'd also like to have a switch which enables this kind of "non-standard" behavior. Actually this should be requested by QEMU, right? So that a single guest can override the CPUID masking done by the kernel if it really really wants to. > > > > (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. I meant to "emulate" such a CPU. -cpu ps4 ;-) Regards, Andre.