Re: [PATCH v2] virtio-balloon-pci: Allow setting nvectors, so we can use MSI-X

[David Hildenbrand]

On 27.01.25 14:11, David Hildenbrand wrote:
> On 15.01.25 17:14, Reza Arbab wrote:
> > Most virtio-pci devices allow MSI-X. Add it to virtio-balloon-pci, but
> > only enable it in new machine types, so we don't break migration of
> > existing machine types between different qemu versions.
> >
> > This copies what was done for virtio-rng-pci in:
> > 9ea02e8f1306 ("virtio-rng-pci: Allow setting nvectors, so we can use MSI-X")
> > bad9c5a5166f ("virtio-rng-pci: fix migration compat for vectors")
> > 62bdb8871512 ("virtio-rng-pci: fix transitional migration compat for vectors")
> >
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
> > ---
>
> This patch enables the virtio-balloon-pci device to be plugged on s390x.
> Before this change, plugging that device would fail with "MSI-X support is
> mandatory on s390".
>
> However, the device does not seem to be initialized by the Linux guest:
>
> # lspci -v
> 0001:00:00.0 Unclassified device [00ff]: Red Hat, Inc. Virtio memory balloon
>           Subsystem: Red Hat, Inc. Device 0005
>           Physical Slot: 00000000
>           Flags: fast devsel, IOMMU group 0
>           Memory at 4001000000000000 (32-bit, non-prefetchable) [virtual] 
> [size=4K]
>           Memory at 4002000000000000 (64-bit, prefetchable) [size=16K]
>           Capabilities: [98] MSI-X: Enable- Count=2 Masked-
>           Capabilities: [84] Vendor Specific Information: VirtIO: <unknown>
>           Capabilities: [70] Vendor Specific Information: VirtIO: Notify
>           Capabilities: [60] Vendor Specific Information: VirtIO: DeviceCfg
>           Capabilities: [50] Vendor Specific Information: VirtIO: ISR
>           Capabilities: [40] Vendor Specific Information: VirtIO: CommonCfg
>
>
> I tried enabling it:
>
> [  343.718529] pci 0001:00:00.0: enabling device (0000 -> 0002)
>
> Without any change. @s390x folks, is there something missing to make it fly,
> or are there additional steps to initialize+use such devices from inside the VM?

Ah! might just be

# CONFIG_VIRTIO_PCI is not set

in my Fedora config,

With that enabled, I think it's working.

--
Cheers,

David / dhildenb