From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:35076) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V1jIS-0002XI-Gt for qemu-devel@nongnu.org; Tue, 23 Jul 2013 16:35:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1V1jIR-0007W7-6b for qemu-devel@nongnu.org; Tue, 23 Jul 2013 16:35:00 -0400 Received: from mail-gh0-x22a.google.com ([2607:f8b0:4002:c05::22a]:43131) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V1jIQ-0007VA-Ua for qemu-devel@nongnu.org; Tue, 23 Jul 2013 16:34:59 -0400 Received: by mail-gh0-f170.google.com with SMTP id z10so2662141ghb.15 for ; Tue, 23 Jul 2013 13:34:58 -0700 (PDT) Sender: Paolo Bonzini Message-ID: <51EEDCCA.600@redhat.com> Date: Tue, 23 Jul 2013 21:43:06 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1374521135-30404-1-git-send-email-ehabkost@redhat.com> <1374521135-30404-3-git-send-email-ehabkost@redhat.com> <51EE4A4B.1040500@redhat.com> <20130723141304.GA3222@otherpad.lan.raisama.net> <51EE9C8E.9030805@redhat.com> <20130723154051.GC3222@otherpad.lan.raisama.net> <51EEADEC.7090700@redhat.com> <20130723174107.GA18526@otherpad.lan.raisama.net> In-Reply-To: <20130723174107.GA18526@otherpad.lan.raisama.net> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [qom-cpu PATCH 2/2] i386: disable PMU passthrough mode by default List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: Igor Mammedov , qemu-devel@nongnu.org, Gleb Natapov , =?ISO-8859-1?Q?Andreas_F=E4rber?= Il 23/07/2013 19:41, Eduardo Habkost ha scritto: > On Tue, Jul 23, 2013 at 06:23:08PM +0200, Paolo Bonzini wrote: >> Il 23/07/2013 17:40, Eduardo Habkost ha scritto: >>> On Tue, Jul 23, 2013 at 05:09:02PM +0200, Paolo Bonzini wrote: >>>> Il 23/07/2013 16:13, Eduardo Habkost ha scritto: >>>>> On Tue, Jul 23, 2013 at 11:18:03AM +0200, Paolo Bonzini wrote: >>>>>> Il 22/07/2013 21:25, Eduardo Habkost ha scritto: >>>>>>> Bug description: QEMU currently gets all bits from GET_SUPPORTED_CPUID >>>>>>> for CPUID leaf 0xA and passes them directly to the guest. This makes >>>>>>> the guest ABI depend on host kernel and host CPU capabilities, and >>>>>>> breaks live migration if we migrate between host with different >>>>>>> capabilities (e.g. different number of PMU counters). >>>>>>> >>>>>>> This patch adds a "pmu-passthrough" property to X86CPU, and set it to >>>>>>> true only on "-cpu host", or on pc-*-1.5 and older machine-types. >>>>>> >>>>>> Can we just call the property "pmu"? It doesn't have to be passthough. >>>>> >>>>> Yes, but the only options we have today are "no PMU" and "passthrough >>>>> PMU". I wouldn't like to make "pmu=on" enable the passthrough behavior >>>>> implicitly (I don't want things that break live-migration to be enabled >>>>> without making it explicit that it is a host-dependent/passthrough >>>>> mode). >>>> >>>> I think "passthrough PMU" should be considered a bug except of course >>>> with "-cpu host". >>>> >>>> If "-cpu Nehalem,pmu=on" goes from passthrough to Nehalem-compatible in >>>> a future QEMU release, that'll be a bugfix. >>> >>> Exactly. But then I don't understand your suggestion. We still need a >>> property to enable pasthrough behavior on old machine-types (not >>> perfect, but a best-effort way to try to keep compatibility), >> >> Do we? >> >> We only need "pmu=on"---which right now is buggy on old machine types >> because it will always passthrough. > > I am not sure I understand what you are arguing for. > > You agree that pmu=on needs to keep the buggy passthrough behavior on > pc-1.5 and older, right? I agree it needs to remain enabled on 1.5. But if, for example, 1.8 makes pmu=on emulate a Nehalem-compatible PMU, I think it is fine if pc-1.5 moves from a host-compatible PMU to a Nehalem-compatible PMU. The reason is that pc-1.5 has never guaranteed any feature of the emulated PMU. Paolo