On Wed, Mar 03, 2021 at 11:12:51AM -0500, Michael S. Tsirkin wrote: > On Wed, Mar 03, 2021 at 09:08:27PM +0800, Jason Wang wrote: > > > > 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 > > okay, please repost. If possible please add code comments > explaining the math.
Nice! Stefan
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