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

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.

We should default to CCW on s390x, though.