On 2021/3/3 6:53 下午, 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).
Yes.
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?
Or it's time to accept this: https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg01330.html Thanks
Stefan
