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
