Re: Inconsistent virtio-net-pci MSI vector count

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 03, 2021 at 10:53:14AM +0000, Stefan Hajnoczi wrote:
> Hi Jason,
> I stumbled across something strange with virtio-net multi-queue today.
> It doesn't seem to be a bug in practice, just an inconsistency. Here are
> the details in case you think something needs to be changed:
> 
> libvirt uses the vectors = 2 * N + 2 formula from
> https://www.linux-kvm.org/page/Multiqueue to calculate the number of PCI
> MSI vectors, where N is the number of rx/tx queue pairs.
> 
> QEMU's virtio-net-pci device has 3 MSI vectors by default. This is
> inconsistent with the formula that libvirt uses (should be 4 instead of
> 3).
> 
> Luckily, the Linux virtio_net.ko driver does not configure a callback
> function for the control virtqueue. Therefore it can still use MSI with
> only 3 vectors (config, rx, tx) instead of 4 (config, rx, tx, ctrl).
> 
> But other driver implementations might need the ctrl vq vector and would
> not have enough MSI vectors.

At a guess control vq can share a vector with config.
Linux is not clever enough to do it yet but it's possible.

> Perhaps new QEMU machine types should set vectors to 4 by default?
> 
> Stefan