Re: Inconsistent virtio-net-pci MSI vector count

["Michael S. Tsirkin" <mst@redhat.com>]

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.

Thanks!

> 
> > 
> > Stefan