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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