From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34491) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fPuFs-0002IN-Tn for qemu-devel@nongnu.org; Mon, 04 Jun 2018 14:30:58 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fPuFp-00043E-Q1 for qemu-devel@nongnu.org; Mon, 04 Jun 2018 14:30:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39606) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fPuFp-00042j-G7 for qemu-devel@nongnu.org; Mon, 04 Jun 2018 14:30:53 -0400 Date: Mon, 4 Jun 2018 15:30:46 -0300 From: Eduardo Habkost Message-ID: <20180604183046.GR7451@localhost.localdomain> References: <20180603092749.107476-1-marcel.apfelbaum@gmail.com> <20180604042928-mutt-send-email-mst@kernel.org> <20180604125415.GN7451@localhost.localdomain> <20180604130129.GG19749@redhat.com> <20180604132624.GO7451@localhost.localdomain> <20180604194913-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20180604194913-mutt-send-email-mst@kernel.org> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , pbonzini@redhat.com, qemu-devel@nongnu.org, rth@twiddle.net On Mon, Jun 04, 2018 at 08:17:23PM +0300, Michael S. Tsirkin wrote: > On Mon, Jun 04, 2018 at 10:26:24AM -0300, Eduardo Habkost wrote: > > On Mon, Jun 04, 2018 at 02:01:29PM +0100, Daniel P. Berrang=E9 wrote: > > > On Mon, Jun 04, 2018 at 09:54:15AM -0300, Eduardo Habkost wrote: > > > > On Mon, Jun 04, 2018 at 04:38:22AM +0300, Michael S. Tsirkin wrot= e: > > > > > On Sun, Jun 03, 2018 at 12:27:49PM +0300, Marcel Apfelbaum wrot= e: > > > > > > Moving to QEMU 3.0 seems like a good opportunity for such a c= hange. > > > > > >=20 > > > > > > I440FX is really old and does not support modern features lik= e IOMMU. > > > > > > Q35's SATA emulation is faster than pc's IDE, native PCI expr= ess hotplug > > > > > > is cleaner than ACPI based one and so on... > > > > > >=20 > > > > > > Also the libvirt guys added very good support for the Q35 mac= hine (thanks!). > > > > > >=20 > > > > > > Management software should always specify the machine type an= d for the > > > > > > current setups, adding '-machine pc' to the command line is n= ot such a > > > > > > big deal. > > > > > >=20 > > > > > > In time the pc machine will fade out and we will probably sto= p adding > > > > > > new versions at some point. > > > > > >=20 > > > > > > Signed-off-by: Marcel Apfelbaum > > > > >=20 > > > > > For command line users, I think changing the default isn't nice= . > > > > >=20 > > > > > Yes it's easy to add -machine pc but there's no documentation > > > > > that tells you to do so. Add to that shortcuts like -cdrom > > > > > stop working, hotplug needs extra bridges to work, and one > > > > > can see that while management tool users benefit from q35, > > > > > command line users will suffer. > > > > >=20 > > > > > Can't we add a tag for management without changing the command = line > > > > > default? How about "management-default"? "recommended"? "latest= "? > > > >=20 > > > > We could add new aliases if they are useful for management > > > > software, but we would need a well-defined use case and set of > > > > requirements+expectations for the new alias. > > >=20 > > > I'm not convinced by the idea of adding a distinct default "for mgm= t". All > > > the problems described wrt 'q35' vs 'pc' apply equally to managemen= t apps > > > as they do to humans. It just happens that one common mgmt layer (l= ibvirt) > > > knows how to handle some of the complexity of q35. Other mgmt apps = though > > > are just as likely to be hurt by the change as humans are. So effec= tively > > > the proposed "for mgmt" is actually "for libvirt >=3D some version= ", which > > > feels like a layering violation to me. > >=20 > > This means the new alias would be used only if requested > > explicitly by management software (not used automatically by > > libvirt). > >=20 > > Taking that into account, I still don't see what exactly would be > > the use case here, and what exactly users can/can't expect when > > using the new alias. >=20 > Let's see what we have now first: >=20 > 1. We have a requirement for the user to save the machine type on insta= ll > and maintain it with the image (a separate thread discusses saving that > as part of a qcow2 image). >=20 > 2. If you use an alias instead you are supposed to resolve it > and save the resolved value. If you save the alias instead, > you can't do cross-version live migration. >=20 > 3. If you don't specify anything you get a machine tagged default. You= are > supposed to find it and save the value found. If you don't and just > keep using the default, you can't do cross-version live migration. >=20 > --- >=20 > So now we would like to relax 3 to say > "If you don't and just keep using the default, some images might not > boot". >=20 > unfortunately we probably can't change 3 like this. > So what I propose instead is simply: >=20 > 4. If you find a machine type value tagged "qmp-default" you must save > the value found. If you don't and just keep using the qmp-default e= ach > time, then existing guest images won't boot. > This relaxed compatibility requirement allows for advanced features > as compared to default. What problems would this system solve? Who would prefer to use the "qmp-default" machine-type instead of the "pc" or "q35" aliases? --=20 Eduardo