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

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.