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

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"?