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

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"> ?