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

On 08.11.19 12:52, Daniel P. Berrangé wrote: > 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. The guest ABI is the same and migration also seems to work. I think your concern boils down to the case that source and target have a different libvirt version (but qemu and kernel and firmware and hardware are identical). So for that case this change would break things if host-model and host-passthrough are not identical. So as of today we have no concern. Now: Would it be a concern if a future QEMU decides to change that equivalence somehow?