Re: [libvirt] Question about the host-model CPU mode

On 10/20/2017 03:43 PM, David Hildenbrand wrote: > On 20.10.2017 15:36, Christian Borntraeger wrote: >> >> >> On 10/20/2017 03:16 PM, David Hildenbrand wrote: >>> >>>> Hi all, >>>> >>>> we recently encountered the problem that the 'host-model' [1] has to be >>>> related to the machine type of a domain. We have following problem: >>>> >>>> Let's assume we've a z13 system with a QEMU 2.9 and we define a >>>> domain using the default s390-virtio-ccw machine together with the >>>> host-model CPU mode [1]. The definition will have the machine >>>> expanded to s390-virtio-ccw-2.9 but retain the host-model CPU mode >>>> in the domain definition. In a next step we upgrade to QEMU 2.10 >>>> (first version to recognize z14). Everything is still fine, even >>>> though the machine runs in 2.9 compatibility mode. Finally we >>>> upgrade to a z14. As a consequence it is not possible to start the >>>> domain anymore as the machine type doesn't support our CPU host >>>> model (which is expanded at start time of the domain). >>> >>> Actually, what is the cause of that problem? I assume it is the gs >>> feature (gs_allowed)? >>> >>> We should really avoid such things (..._allowed) for CPU model features >>> in the future and clue all new such stuff to cpumodel_allowed. >> >> Yes, starting a guest with >> <os> >> <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type> >> </os> >> <cpu mode='host-model'/> >> >> results in >> >> qemu-system-s390x: Some features requested in the CPU model are not available in the configuration: gs >> >> Tying it to cpumodel_allowed would not help, migration-wise though. >> libvirt would still transform >> >> <os> >> <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type> >> </os> >> <cpu mode='host-model'/> > > My point was, that the host model would have to be copied and _remain_ > there when s390-ccw-virtio was expanded to s390-ccw-virtio-2.9. > > So really replacing <cpu mode='host-model'/> by the model z13-base.... > This would at least fix this issue. Just like s390-ccw-virtio get's > replaced and remains thats way. > > But this might for sure have other problems. The problem goes much further. A fresh guest with <os> <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type> </os> <cpu mode='host-model'/> does not start. No migration from an older system is necessary.