On Fri, Nov 08, 2019 at 12:49:23PM +0100, Christian Borntraeger wrote:
>
>
> On 08.11.19 12:43, Daniel P. Berrangé wrote:
> > On Mon, Nov 04, 2019 at 11:49:01AM +0100, David Hildenbrand wrote:
> >> On 02.11.19 11:32, Daniel P. Berrangé wrote:
> >>> On Fri, Nov 01, 2019 at 06:43:16PM +0100, Christian Borntraeger wrote:
> >>>> On the KVM forum I have discussed the default cpu model mode on s390.
> >>>> Right now if the xml does not specify anything, libvirt defaults to
> >>>> not specifying anything on the qemu command line (no -cpu statement)
> >>>> which is the equivalent of -cpu host for s390 which is equivalent to
> >>>> host-passthrough. While this enables all features it does not provide
> >>>> any migration safety by default.
> >>>>
> >>>> So in fact we are kind of "broken" right now when it comes to safery.
> >>>>
> >>>> So we discussed that it would make sense that an empty xml should actually
> >>>> be defaulted to host-model, which results in - as of today - the same guest
> >>>> features but in a migration safe way.
> >>>>
> >>>> There is another change planned right now to actually make the cpu model
> >>>> present in an xml if none was specified. So we could actually do this change
> >>>> before, together or after te other. Jiri and I think it probably makes most
> >>>> sense to have both changes at the same time (in terms of libvirt version).
> >>>>
> >>>> Does anyone see an issue with changing the default model mode to "host-model"
> >>>> if the xml does not specify anything else?
> >>>
> >>> Changing from "host-passthrough" to "host-model" is not a huge difference,
> >>> but it is none the less a guest ABI change. "host-passthrough" doesn't
> >>> provide migration safety in the face of differing hardware, it should still
> >>> be valid for people with homogeneous hardware. So changing the model will
> >>> potentially break some existing usage.
> >>
> >> I guess on s390x this is not the case ("-cpu host", no "-cpu", and passing
> >> the expanded "host" model will result in the same guest ABI, in contrast to
> >> x86 AFAIK). There is this special case, though, where we have old QEMUs
> >> without CPU model support. Not sure how to deal with that, then.
> >
> > I'm still not sure I understand the s390 CPU ABI rules.
> >
> > Current libvirt, no <cpu>, and thus no -cpu.
> >
> > IIUC this is functionally identical to using "-cpu host" and/or
> > <cpu mode="host-passthrough"/>
> >
> > If you are using "-cpu host" / <cpu mode="host-passthrough"> can you
> > live migrate to another host with identical physical CPUs + firmware ?
> >
> >
> > Assuming this is possible, then, can you live migrate a QEMU guest
> > booted with <cpu mode="host-passthrough">, to a QEMU guest booted
> > with <cpu mode="host-model"> ?
>
> Not sure I understand your question. With "can", do you mean "the guest
> has the same guest visible CPU features and types"?
Yes, I mean the migration should succeed from QEMU's POV and additionally
the guest OS should not see any change in CPU ABI exposed from the host.
Regards,
Daniel
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