From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39517) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fHGHf-0004u4-TT for qemu-devel@nongnu.org; Fri, 11 May 2018 18:13:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fHGHc-0000qD-L5 for qemu-devel@nongnu.org; Fri, 11 May 2018 18:13:03 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:57934 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fHGHc-0000q1-G0 for qemu-devel@nongnu.org; Fri, 11 May 2018 18:13:00 -0400 Date: Sat, 12 May 2018 01:12:59 +0300 From: "Michael S. Tsirkin" Message-ID: <20180512010007-mutt-send-email-mst@kernel.org> References: <1523953455-28053-1-git-send-email-wanpengli@tencent.com> <20180417205913.GC29865@localhost.localdomain> <02887fa6-d7c8-bc39-6e65-3c3caec5b903@redhat.com> <20180419195610.GA29865@localhost.localdomain> <98f48c44-e55a-2bd2-0d41-5643f35bce68@redhat.com> <20180419215320.GC29865@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180419215320.GC29865@localhost.localdomain> Subject: Re: [Qemu-devel] [PATCH RESEND v2] i386/kvm: add support for KVM_CAP_X86_DISABLE_EXITS List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: Paolo Bonzini , Wanpeng Li , qemu-devel@nongnu.org, kvm@vger.kernel.org, Radim =?utf-8?B?S3LEjW3DocWZ?= On Thu, Apr 19, 2018 at 06:53:20PM -0300, Eduardo Habkost wrote: > On Thu, Apr 19, 2018 at 11:32:16PM +0200, Paolo Bonzini wrote: > > On 19/04/2018 21:56, Eduardo Habkost wrote: > > > On Thu, Apr 19, 2018 at 05:48:57PM +0200, Paolo Bonzini wrote: > > >> On 17/04/2018 22:59, Eduardo Habkost wrote: > > >>>> + if (disable_exits) { > > >>>> + disable_exits &= (KVM_X86_DISABLE_EXITS_MWAIT | > > >>>> + KVM_X86_DISABLE_EXITS_HLT | > > >>>> + KVM_X86_DISABLE_EXITS_PAUSE); > > >>>> + if (env->user_features[FEAT_KVM] & KVM_PV_UNHALT) { > > >>>> + disable_exits &= ~KVM_X86_DISABLE_EXITS_HLT; > > >>>> + } > > >>> > > >>> In the future, if we decide to enable kvm-pv-unhalt by default, > > >>> should "-cpu ...,kvm-hint-dedicated=on" disable kvm-pv-unhalt > > >>> automatically, or should we require an explicit > > >>> "kvm-hint-dedicated=on,kvm-pv-unhalt=off" option? > > >> > > >> It should be automatic. > > >> > > >>> For today's defaults, this patch solves the problem, only one > > >>> thing is missing before I give my R-b: we need to clearly > > >>> document what exactly are the consequences and requirements of > > >>> setting kvm-hint-dedicated=on (I'm not sure if the best place for > > >>> this is qemu-options.hx, x86_cpu_list(), or somewhere else). > > >> > > >> I don't think we have a good place for this kind of documentation, > > >> unfortunately. Right now it is mentioned in > > >> Documentation/virtual/kvm/cpuid.txt. > > > > > > With this patch, the QEMU option will do more than just setting > > > the CPUID bit, that's why I miss more detailed documentation on > > > the QEMU side. But I agree we have no obvious place for that > > > documentation. > > > > > > In the worst case we can just add a code comment on top of > > > feature_word_info[FEAT_KVM_HINTS].feat_names warning that > > > kvm-hint-dedicated won't just enable the flag on CPUID and has > > > other side-effects. > > > > Maybe we should use "-realtime dedicated=on" instead of, or in addition > > to kvm-hint-dedicated=on? > > Maybe it's a better idea than overloading an option that is only > expected to control a CPUID bit. Well -realtime would be a bit confusing in that it enables mlock by default. >>From pure API point of view, hint-dedicated looks good since it seems to say "optimize for a dedicated host CPU" and it's a hint in that guests keep working if you violate this slightly once in a while. But I agree there's a problem: right now "kvm-" means "KVM PV" as opposed to e.g. HV enlightened gusts. So if you enable hyperv and also want to disable halt existing, what then? What should kvm-hint-dedicated=on do? So how about a new hint-dedicated=on cpu flag? We can have that set kvm-hint-dedicated if kvm PV is enabled. > -- > Eduardo