Re: [RFC 1/2] virtio_net: timeout control virtqueue commands

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

On Fri, Oct 17, 2025 at 09:21:17AM +0200, Eugenio Perez Martin wrote:
> On Fri, Oct 17, 2025 at 8:39 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Fri, Oct 17, 2025 at 08:36:41AM +0200, Eugenio Perez Martin wrote:
> > > On Thu, Oct 16, 2025 at 8:25 AM Eugenio Perez Martin
> > > <eperezma@redhat.com> wrote:
> > > >
> > > > On Thu, Oct 16, 2025 at 8:22 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > >
> > > > > On Thu, Oct 16, 2025 at 02:03:57PM +0800, Jason Wang wrote:
> > > > > > On Thu, Oct 16, 2025 at 1:45 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > > >
> > > > > > > On Thu, Oct 16, 2025 at 01:39:58PM +0800, Jason Wang wrote:
> > > > > > > > > >
> > > > > > > > > > Not exactly bufferize, record.  E.g. we do not need to send
> > > > > > > > > > 100 messages to enable/disable promisc mode - together they
> > > > > > > > > > have no effect.
> > > > > > > >
> > > > > > > > Note that there's a case that multiple commands need to be sent, e.g
> > > > > > > > set rx mode. And assuming not all the commands are the best effort,
> > > > > > > > kernel VDUSE still needs to wait for the usersapce at least for a
> > > > > > > > while.
> > > > > > >
> > > > > > > Not wait, record. Generate 1st command, after userspace consumed it -
> > > > > > > generate and send second command and so on.
> > > > > >
> > > > > > Right, that's what I asked in another thread, we still need a timeout
> > > > > > here.
> > > > >
> > > > > we do not need a timeout.
> > > > >
> > > > > > Then I think it would not be too much difference whether it is
> > > > > > VDUSE or CVQ that will fail or break the device. Conceptually, VDUSE
> > > > > > can only advertise NEEDS_RESET since it's a device implementation.
> > > > > > VDUSE can not simply break the device as it requires synchronization
> > > > > > which is not easy.
> > > > > >
> > > > > > > But for each bit of data, at most one command has to be sent,
> > > > > > > we do not care if guest tweaked rx mode 3 times, we only care about
> > > > > > > the latest state.
> > > > > >
> > > > > > Yes, but I want to know what's best when VDUSE meets userspace timeout.
> > > > > >
> > > > > > Thanks
> > > > >
> > > > >
> > > > > userspace should manage its own timeouts.
> > > > >
> > > >
> > > > Can we just apply the patch 2/2 of this RFC directly and apply the
> > > > VDUSE CVQ on top then? What are we missing to do it?
> > > >
> > >
> > > Even better, can we just revert commit 56e71885b0349 ("vduse:
> > > Temporarily fail if control queue feature requested") ?
> >
> > No because both would let userspace hang kernels merely by
> > not consuming buffers.
> >
> 
> My understanding was that you want to be able to debug qemu with gdb
> from that hang [1].
> 
> Could you put an example of the whole chain of events you expect? From
> the moment the driver sends a VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command,
> the VDUSE CVQ Forwards the command to the VDUSE device in the
> userspace, and then the vduse userland device does not reply.

this is not the idea.

the idea is that kernel handles VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET.

separately it notifies userspace that some configuration
changed and userspace gets the new value.
Or not, if it is stuck.




> How does the VDUSE CVQ detect that the VDUSE device implemented in
> userland does not reply? What are the next steps from that point of
> the kernel VDUSE module?
> 
> Thanks!
> 
> [1] https://lore.kernel.org/lkml/20251014042459-mutt-send-email-mst@kernel.org/