On Mon, Nov 01, 2021 at 04:42:01PM +0100, Eugenio Perez Martin wrote: > On Mon, Nov 1, 2021 at 9:58 AM Eugenio Perez Martin <epere...@redhat.com> > wrote: > > > > On Mon, Nov 1, 2021 at 4:34 AM Jason Wang <jasow...@redhat.com> wrote: > > > > > > On Fri, Oct 29, 2021 at 10:16 PM Eugenio Pérez <epere...@redhat.com> > > > wrote: > > > > > > > > The -1 assumes that all devices with no cvq have an spare vq allocated > > > > for them, but with no offer of VIRTIO_NET_F_CTRL_VQ. This may not be the > > > > case, and the device may have a pair number of queues. > > > > > > > > To fix this, just resort to the lower even number of queues. > > > > > > > > Fixes: 049eb15b5fc9 ("vhost: record the last virtqueue index for the > > > > virtio device") > > > > Signed-off-by: Eugenio Pérez <epere...@redhat.com> > > > > --- > > > > hw/net/vhost_net.c | 2 +- > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c > > > > index 0d888f29a6..edf56a597f 100644 > > > > --- a/hw/net/vhost_net.c > > > > +++ b/hw/net/vhost_net.c > > > > @@ -330,7 +330,7 @@ int vhost_net_start(VirtIODevice *dev, > > > > NetClientState *ncs, > > > > NetClientState *peer; > > > > > > > > if (!cvq) { > > > > - last_index -= 1; > > > > + last_index &= ~1ULL; > > > > } > > > > > > The math here looks correct but we need to fix vhost_vdpa_dev_start() > > > instead? > > > > > > if (dev->vq_index + dev->nvqs - 1 != dev->last_index) { > > > ... > > > } > > > > > > > If we just do that, devices that offer an odd number of queues but do > > not offer ctrl vq would never enable the last vq pair, isn't it? > > > > To expand the issue, > > With that condition it is not possible to make vp_vdpa work on devices > with no cvq. If I set the L0 guest's device with no cvq (with -device > virtio-net-pci,...,ctrl_vq=off,mq=off). The nested VM will enter that > conditional in vhost_net_start, and will mark last_index=1, making it > impossible to start a vhost_vdpa device. > > However, re-reading the standard: > > controlq only exists if VIRTIO_NET_F_CTRL_VQ set. > > So the code is actually handling an invalid device: The device set > VIRTIO_NET_F_CTRL_VQ but offered an odd number of VQs. > > Do we have an example of such a device? It's not the case on qemu > virtio-net, with or without vhost-net in L0 device. The operation &= > ~1ULL is an intended noop in case the queues are already even. I'm > fine to keep making last_index even, so we have that safety net, with > further clarifications as MST said, just in case the device is not > behaving well. But maybe it's even better just to delete that > conditional entirely? > > Thanks! >
For sure, no need to handle an invalid configuration. Do you have a patch in mind? It'd be easier to discuss things with a specific patch rather than theoretically. > > > > Also, I would say that the right place for the solution of this > > problem should not be virtio/vhost-vdpa: This is highly dependent on > > having cvq, and this implies a knowledge about the use of each > > virtqueue. Another kind of device could have an odd number of > > virtqueues naturally, and that (-1) would not work for them, isn't it? > > > > Thanks! > > > > > Thanks > > > > > > > > > > > if (!k->set_guest_notifiers) { > > > > -- > > > > 2.27.0 > > > > > > >