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Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-versi
From: |
Andrea Bolognani |
Subject: |
Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-version setting to "max" |
Date: |
Wed, 11 Apr 2018 17:35:55 +0200 |
On Tue, 2018-04-10 at 09:52 +0100, Daniel P. Berrangé 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.
>
> Libvirt uses versioned machine types and does not specify -cpu unless the
> user has added <cpu> 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 <cpu> 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.
--
Andrea Bolognani / Red Hat / Virtualization