Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932655AbaFSMRu (ORCPT ); Thu, 19 Jun 2014 08:17:50 -0400 Received: from mx1.redhat.com ([209.132.183.28]:32944 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757919AbaFSMRs (ORCPT ); Thu, 19 Jun 2014 08:17:48 -0400 Date: Thu, 19 Jun 2014 15:17:56 +0300 From: "Michael S. Tsirkin" To: Nadav Amit Cc: Gleb Natapov , "Gabriel L. Somlo" , Eric Northup , Nadav Amit , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , the arch/x86 maintainers , Linux Kernel Mailing List , KVM , joro@8bytes.org, agraf@suse.de Subject: Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation Message-ID: <20140619121756.GA28523@redhat.com> References: <20140618184601.GE1695@ERROL.INI.CMU.EDU> <20140619101811.GA5777@redhat.com> <1B06E887-9D07-4E85-AE06-75B01787C488@gmail.com> <20140619112356.GB429@minantech.com> <53A2CEF4.3050902@gmail.com> <20140619120739.GA7289@minantech.com> <53A2D32D.8020305@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53A2D32D.8020305@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 19, 2014 at 03:10:21PM +0300, Nadav Amit wrote: > On 6/19/14, 3:07 PM, Gleb Natapov wrote: > >On Thu, Jun 19, 2014 at 02:52:20PM +0300, Nadav Amit wrote: > >>On 6/19/14, 2:23 PM, Gleb Natapov wrote: > >>>On Thu, Jun 19, 2014 at 01:53:36PM +0300, Nadav Amit wrote: > >>>> > >>>>On Jun 19, 2014, at 1:18 PM, Michael S. Tsirkin wrote: > >>>> > >>>>>On Wed, Jun 18, 2014 at 02:46:01PM -0400, Gabriel L. Somlo wrote: > >>>>>>On Wed, Jun 18, 2014 at 10:59:14AM -0700, Eric Northup wrote: > >>>>>>>On Wed, Jun 18, 2014 at 7:19 AM, Nadav Amit wrote: > >>>>>>>>mwait and monitor are currently handled as nop. Considering this behavior, they > >>>>>>>>should still be handled correctly, i.e., check execution conditions and generate > >>>>>>>>exceptions when required. mwait and monitor may also be executed in real-mode > >>>>>>>>and are not handled in that case. This patch performs the emulation of > >>>>>>>>monitor-mwait according to Intel SDM (other than checking whether interrupt can > >>>>>>>>be used as a break event). > >>>>>>>> > >>>>>>>>Signed-off-by: Nadav Amit > >>>>>> > >>>>>>How about this instead (details in the commit log below) ? Please let > >>>>>>me know what you think, and if you'd prefer me to send it out as a > >>>>>>separate patch rather than a reply to this thread. > >>>>>> > >>>>>>Thanks, > >>>>>>--Gabriel > >>>>> > >>>>>If there's an easy workaround, I'm inclined to agree. > >>>>>We can always go back to Gabriel's patch (and then we'll need > >>>>>Nadav's one too) but if we release a kernel with this > >>>>>support it becomes an ABI and we can't go back. > >>>>> > >>>>>So let's be careful here, and revert the hack for 3.16. > >>>>> > >>>>> > >>>>>Acked-by: Michael S. Tsirkin > >>>>> > >>>>Personally, I got a custom guest which requires mwait for executing correctly. > >>>Can you elaborate on this guest a little bit. With nop implementation > >>>for mwait the guest will hog a host cpu. Do you consider this to be > >>>"executing correctly?" > >>> > >>>-- > >> > >>mwait is not as "clean" as it may appear. It encounters false wake-ups due > >>to a variety of reasons, and any code need to recheck the wake-up condition > >>afterwards. Actually, some CPUs had bugs that caused excessive wake-ups that > >>degraded performance considerably (Nehalem, if I am not mistaken). > >>Therefore, handling mwait as nop is logically correct (although it may > >>degrade performance). > >> > >>For the reference, if you look at the SDM 8.10.4, you'll see: > >>"Multiple events other than a write to the triggering address range can > >>cause a processor that executed MWAIT to wake up. These include events that > >>would lead to voluntary or involuntary context switches, such as..." > >> > >>Note the words "include" in the sentence "These include events". Software > >>has no way of controlling whether it gets false wake-ups and cannot rely on > >>the wake-up as indication to anything. > >> > >That's all well and good and I didn't say that nop is not a valid > >mwait implementation, it is, though there is a big difference between > >"encounters false wake-ups" and never sleeps. What I asked is do you > >consider your guest hogging host cpu to be "executing correctly?". What > >this guest is doing that such behaviour is tolerated and shouldn't it > >be better to just poll for a condition you are waiting for instead of > >executing expensive vmexits. This will also hog 100% host cpu, but will > >be actually faster. > > > You are correct, but unfortunately I have no control over the guest > workload. In this specific workload I do not care about performance but only > about correctness. > > Nadav No one prevents you from patching your kernel to run this workload. But is this of use to anyone else? If yes why? -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/