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

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

On Wed, Oct 15, 2025 at 10:03:49AM +0200, Maxime Coquelin wrote:
> On Wed, Oct 15, 2025 at 9:45 AM Eugenio Perez Martin
> <eperezma@redhat.com> wrote:
> >
> > On Wed, Oct 15, 2025 at 9:05 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > 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.
> > >
> >
> > Let me take a step back, I think I didn't describe the scenario well enough.
> >
> > We have a VDUSE device, and then the same host is interacting with the
> > device through the virtio_net driver over virtio_vdpa.
> >
> > Then, the virtio_net driver sends a control command though its CVQ, so
> > it *takes the RTNL*. That command reaches the VDUSE CVQ.
> >
> > It does not matter if the VDUSE device in the userland processes the
> > commands through a CVQ, reading the vduse character device, or another
> > system. The question is: what to do if the VDUSE device does not
> > process that command in a timely manner? Should we just let the RTNL
> > be taken forever?
> >
> 
> My understanding is that:
> 1. Virtio-net sends a control messages, waits for reply
> 2. VDUSE driver dequeues it, adds it to the SCVQ, replies OK to the CVQ
> 3. Userspace application dequeues the message from the SCVQ
>  a. If handling is successful it replies OK
>  b. If handling fails, replies ERROR
> 4. VDUSE driver reads the reply
>  a. if OK, do nothing
>  b. if ERROR, mark the device as broken?
> 
> This is simplified as it does not take into account SCVQ overflow if
> the application is stuck.
> If IIUC, Michael suggests to only enqueue a single message at the time
> in the SVQ,
> and bufferize the pending messages in the VDUSE driver.

Not exactly bufferize, record.  E.g. we do not need to send
100 messages to enable/disable promisc mode - together they
have no effect.

-- 
MST