From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755775AbdKCKKE (ORCPT ); Fri, 3 Nov 2017 06:10:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59001 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750821AbdKCKKB (ORCPT ); Fri, 3 Nov 2017 06:10:01 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 600FB25C46 Authentication-Results: ext-mx06.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx06.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=pbonzini@redhat.com Subject: Re: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set To: Eduardo Valentin Cc: rkrcmar@redhat.com, Matt Wilson , Jonathan Corbet , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Peter Zijlstra , Waiman Long , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, "Jan H . Schoenherr" , Anthony Liguori References: <1509644731-2249-1-git-send-email-eduval@amazon.com> <6bd9aee8-1d9e-d088-d13d-653c0628b394@redhat.com> <20171102180854.GA27676@u40b0340c692b58f6553c.ant.amazon.com> <39241f1f-1b88-709e-21b1-2019ad156b08@redhat.com> <20171102184311.GC27676@u40b0340c692b58f6553c.ant.amazon.com> From: Paolo Bonzini Message-ID: Date: Fri, 3 Nov 2017 11:09:53 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <20171102184311.GC27676@u40b0340c692b58f6553c.ant.amazon.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Fri, 03 Nov 2017 10:10:01 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/11/2017 19:43, Eduardo Valentin wrote: > On Thu, Nov 02, 2017 at 07:24:16PM +0100, Paolo Bonzini wrote: >> On 02/11/2017 19:08, Eduardo Valentin wrote: >>> On Thu, Nov 02, 2017 at 06:56:46PM +0100, Paolo Bonzini wrote: >>>> On 02/11/2017 18:45, Eduardo Valentin wrote: >>>>> Currently, the existing qspinlock implementation will fallback to >>>>> test-and-set if the hypervisor has not set the PV_UNHALT flag. >>>>> >>>>> This patch gives the opportunity to guest kernels to select >>>>> between test-and-set and the regular queueu fair lock implementation >>>>> based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED >>>>> flag is not set, the code will still fall back to test-and-set, >>>>> but when the PV_DEDICATED flag is set, the code will use >>>>> the regular queue spinlock implementation. >>>> >>>> Have you seen Waiman's series that lets you specify this on the guest >>>> command line instead? Would this be acceptable for your use case? >>> >>> No, can you please share a link to it? is it already merged to tip/master? >> >> [PATCH-tip v2 0/2] x86/paravirt: Enable users to choose PV lock type >> https://lkml.org/lkml/2017/11/1/655 >> >>>> (In other words, is there a difference for you between making the host >>>> vs. guest administrator toggle the feature? "@amazon.com" means you are >>>> the host admin, how would you use it?) >>> >>> The way I think of this is this is a flag set by host side so the >>> guest adapts accordingly. >>> >>> If the admin in guest side wants to ignore what the host is >>> flagging, that is a different story. >> >> Okay, this makes sense. But perhaps it should be a separate CPUID leaf, >> such as "configuration hints", rather than properly a feature. > > Oh OK, you don't think this starts to deviate from the feature concept. > But would the PV_UNHALT also go to "configuration hints" bucket? PV_UNHALT says whether the pvqspinlock API is available, PV_DEDICATED says whether it should be used. > Another way to see this is we have three locking feature options to select from, > so we need at least two bits here. PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock PV_DEDICATED = 0, PV_UNHALT = 1: default is pvqspinlock PV_DEDICATED = 0, PV_UNHALT = 0: default is tas What do you think? Paolo