Inconsistent virtio-net-pci MSI vector count

[Stefan Hajnoczi]

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.

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

Stefan

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