Re: [PATCH 7/8] qemuxmlconftest: Update memory-hotplug-virtio-mem-s390x.xml

On 1/27/25 15:15, David Hildenbrand wrote: > On 27.01.25 14:41, Michal Prívozník wrote: >> On 1/27/25 13:44, David Hildenbrand wrote: >>> On 27.01.25 13:01, Boris Fiuczynski wrote: >>>> On 1/24/25 13:35, David Hildenbrand wrote: >>>>> On 24.01.25 13:21, Michal Privoznik wrote: >>>>>> Drop explicit request to place virtio-mem on PCI bus from the >>>>>> input memory-hotplug-virtio-mem-s390x.xml and demonstrate how the >>>>>> device is automatically placed onto CCW. >>>>>> >>>>> >>>>> Could it still be manually placed on the PCI bus? >> >> It can, but as you point out, qemu refuses to start. >> >>>>> >>>>> As of now, virtio-mem-pci is not supported on s390x -- IIRC >>>>> plugging the >>>>> device would fail --  but maybe, in a distant future it might be >>>>> supported. >>>>> >>>> >>>> David, >>> >>> Hi Boris, >>> >>>> the libvirt probing of capabilities with qemu v9.2.0-1203-gd6430c17d7 >>>> returns virtio-mem-pci support based on the QOM. Should that be fixed? >>> >>> Right, it's similar to virtio-balloon-pci: while it is compiled into >>> QEMU, plugging these devices will fail due to lack of MSI-X support. [1] >>> >>> For virtio-mem-pci, in addition to MSI-X support, we'll have to wire up >>> the (un)plug handlers in the machine, which are currently blocked: >>> >>> Which leaves us with three options: >>> >>> (1) Leave it as is: device is compiled in but cannot be used, just like >>>      virtio-balloon-pci >>> (2) Implement MSI-X and (un)plug support >>> (3) Do not compile the device in >> >> This last option is something I've experimented with, but then got lost >> in the weeds of code interdeps. > > Yes, that last thing happened to me when I last looked into that ... > >> In the end I've decided it's not a huge >> problem, is it. In my patches (e.g. 5/8) I look what bus is virtio-mem >> attached to and check for corresponding capability. >> >> OTOH, support for .prealloc and .dynamic-memslots is detected from >> virtio-mem-pci device. If virtio-mem-pci device would be compiled out >> then libvirt would need a code to check these attributes for virtio- >> mem-ccw. > > Yes, that would be unfortunate. > > Looks like I have it running, so we can just support it in QEMU as it > seems. I see you already posted patches. Let me take a look. > We should default to CCW on s390x, though. Absolutely! And that's what my patches do too. Unless the device is explicitly placed onto PCI bus (<address type='pci'/>) libvirt puts it onto CCW, which in turn selects virtio-mem-ccw when building cmd line.