On Wed, Apr 11, 2018 at 05:35:55PM +0200, Andrea Bolognani wrote: > 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.
That model ID string is tied to the machine type, so it is stable. Older machine types use PC_CPU_MODEL_IDS() in pc.h to ensure they have not changed. > 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. Provided you have picked a versioned machine type, it is expected that all other defaults are stable, and that includes CPU models. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|