From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36607) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fYpAH-0001fy-CB for qemu-devel@nongnu.org; Fri, 29 Jun 2018 04:54:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fYpAE-0003wi-7k for qemu-devel@nongnu.org; Fri, 29 Jun 2018 04:54:01 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:60974 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 1fYpAE-0003u4-1p for qemu-devel@nongnu.org; Fri, 29 Jun 2018 04:53:58 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BB28587903 for ; Fri, 29 Jun 2018 08:53:56 +0000 (UTC) Date: Fri, 29 Jun 2018 09:53:53 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20180629085353.GA5301@work-vm> References: <20180628154502.GO3513@redhat.com> <20180628195227.GH7451@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20180628195227.GH7451@localhost.localdomain> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] CPU model versioning separate from machine type versioning ? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , libvir-list@redhat.com, qemu-devel@nongnu.org * Eduardo Habkost (ehabkost@redhat.com) wrote: > On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrang=E9 wrote: > [...] > > What if we can borrow the concept of versioning from machine types an= d apply > > it to CPU models directly. For example, considering the history of "H= aswell" > > in QEMU, if we had versioned things, we would by now have: > >=20 > > Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185d= e3ab60a) > > Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a74= 2a85212c_ > > Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad= 2acb05e6da03753) > > Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60a= d8b11 > > Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b6= 86e31ab87458) > > Haswell-3.0.0 - added 'ssbd' (never done) > >=20 > > If we followed the machine type approach, then a bare "Haswell" would > > statically resolve at build time to the most recent Haswell-X.X.X ver= sion > > associated with the QEMU release. This is unhelpful as we have a dire= ct > > dependancy on the host hardware features. Better would be for a bare > > "Haswell" to be dynamically resolved at runtime, picking the most rec= ent > > version that is capable of launching given the current hardware, KVM/= TCG impl > > and QEMU version. > >=20 > > ie -cpu Haswell > >=20 > > should use Haswell-2.5.0 if on silicon with the TSX errata applied, > > but use Haswell-2.12.0 if the Spectre errata is applied in microcode, > > and use Haswell-3.0.0 once Intel finally releases SSBD microcode erra= ta. >=20 > Doing this unconditionally would make > "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and > break existing usage. But this behavior could be enabled > explicitly somehow. >=20 > >=20 > > Versioning of CPU models as opposed to using arbitrary string suffixe= s > > (-noTSX, -IBRS) has a number of usability improvements that we would > > gain with versioned machine types, while avoiding exploding the machi= ne > > type matrix. With versioned CPU models we can > >=20 > > - Automatically tailor the best model based on hardware support > >=20 > > - Users always get the best model if they use the bare CPU name > >=20 > > - It is obvious to users which is the "best" / "newest" CPU model > >=20 > > - Avoid combinatorial expansion of machines since same CPU model > > version can be added to all releases without adding machine types. > >=20 > > - Users can still force a specific downgraded model by using the > > fully versioned name. > >=20 > > Such versioning of CPU models would largely "just work" with existing > > libvirt versions, but to libvirt would really want to expand the bare > > CPU name to a versioned CPU name when recording new guest XML, so the > > ABI is preserved long term. > >=20 > > An application like virt-manager which wants a simple UI can forever = be > > happy simply giving users a list of bare CPU model names, and allowin= g > > libvirt / QEMU to automatically expand to the best versioned model fo= r > > their host. > >=20 > > An application like oVirt/OpenStack which wants direct control can al= low > > the admin to choice if a bare name, or explicitly picking a versioned= name > > if they need to cope with possibility of outdated hosts. > >=20 >=20 > The proposal makes sense, and I think most of it can be already > implemented on top of existing query-cpu-model-* commands. > query-cpu-model-expansion type=3Dstatic can expand to a versioned > CPU model. >=20 > We will probably need to make query-cpu-model-expansion accept a > machine-type name as input, and/or add a new flag meaning "please > give me the best CPU version you have, not the one defined by the > current machine-type". >=20 > I'm not sure what would be the best way to encode two types of > information, though: >=20 Both of those are solved with the numbering scheme > * Fallback/alternatives info, e.g.: "It makes sense to use > Haswell-{3.0,2.12,2.5,...} if Haswell-3.1 is not runnable and the > user asked for Haswell". Use the highest that works. > * Ordering/preference info, e.g.: "Haswell-3.1 is better than > Haswell-3.0, prefer the latter" Higher is better. The only thing that worries me about a numbering scheme is that it's now more difficult for a user to know whether they've got the type with a fix for a particular vulnerability. We're going to have to say something like: 'For the new XYZ vulnerability make sure you're using Haswell-3.2 or later, SkyLake-2.6 or later, Westmere-4.8 or later .....' which all gets a bit confusing. Dave > --=20 > Eduardo >=20 -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK