From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33310) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1f6Hn5-0006WQ-Ed for qemu-devel@nongnu.org; Wed, 11 Apr 2018 11:36:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1f6Hn4-0001Na-Ij for qemu-devel@nongnu.org; Wed, 11 Apr 2018 11:36:07 -0400 Message-ID: <1523460955.2942.5.camel@redhat.com> From: Andrea Bolognani Date: Wed, 11 Apr 2018 17:35:55 +0200 In-Reply-To: <20180410085200.GD5155@redhat.com> References: <20180409154921.29906-1-wei@redhat.com> <20180409155634.GO18283@redhat.com> <6bb12858-3b4e-e92f-93ea-b8708c8be870@redhat.com> <1523346093.16671.23.camel@redhat.com> <20180410085200.GD5155@redhat.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-version setting to "max" List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Daniel P. =?ISO-8859-1?Q?Berrang=E9?=" Cc: Wei Huang , qemu-devel@nongnu.org, peter.maydell@linaro.org, drjones@redhat.com, qemu-arm@nongnu.org On Tue, 2018-04-10 at 09:52 +0100, Daniel P. Berrang=C3=A9 wrote: > On Tue, Apr 10, 2018 at 09:41:33AM +0200, Andrea Bolognani wrote: > > I figure the people not explicitly specifying a CPU model on the > > command line will probably also use '-M virt' instead of versioned > > machine types, which means they will get a different guest behavior > > after upgrading QEMU regardless. >=20 > Libvirt uses versioned machine types and does not specify -cpu unless t= he > user has added to their XML. IOW libvirt assumes the default CPU > model is stable because that's what QEMU has promised in the past. Hm, you have a point. I wonder how well that works in practice, though. I started a guest with no element on my laptop and it ended up having vendor_id : GenuineIntel cpu family : 6 model : 6 model name : QEMU Virtual CPU version 2.5+ stepping : 3 which I guess translates to the qemu64 CPU model, based on the description. I have verified the -cpu option is not present on the command line. The name seems to imply that if I were using a QEMU release older than 2.5 I would get a different CPU model, but maybe the stable CPU guarantee you mention is just a fairly recent development. I also know that ppc64 performs some trickery if you don't specify a CPU model, so by default you get a behavior which is pretty close to using -cpu host. Basically I'm wondering how reasonable it is to expect a migratable machine and a stable guest ABI when relying on QEMU defaults instead of explicitly picking a CPU model. --=20 Andrea Bolognani / Red Hat / Virtualization