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

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

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.

-- 
MST