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

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

On Wed, Oct 15, 2025 at 08:52:50AM +0200, Eugenio Perez Martin wrote:
> On Wed, Oct 15, 2025 at 8:33 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Oct 15, 2025 at 08:08:31AM +0200, Eugenio Perez Martin wrote:
> > > On Tue, Oct 14, 2025 at 11:25 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Tue, Oct 14, 2025 at 11:14:40AM +0200, Maxime Coquelin wrote:
> > > > > On Tue, Oct 14, 2025 at 10:29 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > >
> > > > > > On Tue, Oct 07, 2025 at 03:06:21PM +0200, Eugenio Pérez wrote:
> > > > > > > An userland device implemented through VDUSE could take rtnl forever if
> > > > > > > the virtio-net driver is running on top of virtio_vdpa.  Let's break the
> > > > > > > device if it does not return the buffer in a longer-than-assumible
> > > > > > > timeout.
> > > > > >
> > > > > > So now I can't debug qemu with gdb because guest dies :(
> > > > > > Let's not break valid use-cases please.
> > > > > >
> > > > > >
> > > > > > Instead, solve it in vduse, probably by handling cvq within
> > > > > > kernel.
> > > > >
> > > > > Would a shadow control virtqueue implementation in the VDUSE driver work?
> > > > > It would ack systematically messages sent by the Virtio-net driver,
> > > > > and so assume the userspace application will Ack them.
> > > > >
> > > > > When the userspace application handles the message, if the handling fails,
> > > > > it somehow marks the device as broken?
> > > > >
> > > > > Thanks,
> > > > > Maxime
> > > >
> > > > Yes but it's a bit more convoluted  than just acking them.
> > > > Once you use the buffer you can get another one and so on
> > > > with no limit.
> > > > One fix is to actually maintain device state in the
> > > > kernel, update it, and then notify userspace.
> > > >
> > >
> > > I thought of implementing this approach at first, but it has two drawbacks.
> > >
> > > The first one: it's racy. Let's say the driver updates the MAC filter,
> > > VDUSE timeout occurs, the guest receives the fail, and then the device
> > > replies with an OK. There is no way for the device or VDUSE to update
> > > the driver.
> >
> > There's no timeout. Kernel can guarantee executing all requests.
> >
> 
> I don't follow this. How should the VDUSE kernel module act if the
> VDUSE userland device does not use the CVQ buffer then?

First I am not sure a VQ is the best interface for talking to userspace.
But assuming yes - just avoid sending more data, send it later after
userspace used the buffer.


> >
> >
> > >
> > > The second one, what to do when the VDUSE cvq runs out of descriptors?
> > > While the driver has its descriptor returned with VIRTIO_NET_ERR, the
> > > VDUSE CVQ has the descriptor available. If this process repeats to
> > > make available all of the VDUSE CVQ descriptors, how can we proceed?
> >
> > There's no reason to return VIRTIO_NET_ERR ever and cvq will not run
> > out of descriptors. Kernel uses cvq buffers.
> >
> >
> > > I think both of them can be solved with the DEVICE_NEEDS_RESET status
> > > bit, but it is not implemented in the drivers at this moment.
> >
> > No need for a reset, either.
> >