Re: [libvirt] s390: change default cpu model to host-model?

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.