On Tue, Oct 28, 2025 at 03:37:09PM +0100, Eugenio Perez Martin wrote: > On Tue, Oct 28, 2025 at 3:10 PM Michael S. Tsirkin <[email protected]> wrote: > > > > On Wed, Oct 22, 2025 at 02:55:18PM +0200, Eugenio Perez Martin wrote: > > > On Wed, Oct 22, 2025 at 1:43 PM Michael S. Tsirkin <[email protected]> > > > wrote: > > > > > > > > On Wed, Oct 22, 2025 at 12:50:53PM +0200, Eugenio Perez Martin wrote: > > > > > Let me switch to MQ as I think it illustrates the point better. > > > > > > > > > > IIUC the workflow: > > > > > a) virtio-net sends MQ_VQ_PAIRS_SET 2 to the device > > > > > b) VDUSE CVQ sends ok to the virtio-net driver > > > > > c) VDUSE CVQ sends the command to the VDUSE device > > > > > d) Now the virtio-net driver sends virtio-net sends MQ_VQ_PAIRS_SET 1 > > > > > e) VDUSE CVQ sends ok to the virtio-net driver > > > > > > > > > > The device didn't process the MQ_VQ_PAIRS_SET 1 command at this point, > > > > > so it potentially uses the second rx queue. But, by the standard: > > > > > > > > > > The device MUST NOT queue packets on receive queues greater than > > > > > virtqueue_pairs once it has placed the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET > > > > > command in a used buffer. > > > > > > > > > > So the driver does not expect rx buffers on that queue at all. From > > > > > the driver's POV, the device is invalid, and it could mark it as > > > > > broken. > > > > > > > > ok intresting. Note that if userspace processes vqs it should process > > > > cvq too. I don't know what to do in this case yet, I'm going on > > > > vacation, let me ponder this a bit. > > > > > > > > > > Sure. > > > > So let me ask you this, how are you going to handle device reset? > > Same issue, it seems to me. > > > > Well my proposal is to mark it as broken so it needs to be reset > manually.
Heh but guest assumes after reset device does not poke at guest memory, and will free up and reuse that memory. If userspace still pokes at it -> plus plus ungood. > For example, unbinding and binding the driver in Linux. The > point is that the driver cannot trust the device anymore as it is in > an invalid state. Maybe suspend and reset all the vqs is also a valid > solution to un-broke it if the device supports it but I think a race > is unavoidable there, and I'm not sure how to communicate it to > userspace for all kinds of devices. Incrementing rx errors could be a > first proposal. > > If we want to track it in VDUSE we should implement NEEDS_RESET and > leave all the old drivers without solution. That's why I think it is > better to solve all the problems at once in the driver.
