Re: [RFC PATCH 13/27] vhost: Send buffers to device

[Stefan Hajnoczi]

On Mon, Mar 22, 2021 at 04:55:13PM +0100, Eugenio Perez Martin wrote:
> On Mon, Mar 22, 2021 at 11:51 AM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > On Thu, Mar 11, 2021 at 07:53:53PM +0100, Eugenio Perez Martin wrote:
> > > On Fri, Jan 22, 2021 at 7:18 PM Eugenio Perez Martin
> > > <eperezma@redhat.com> wrote:
> > > >
> > > > On Thu, Dec 10, 2020 at 12:55 PM Stefan Hajnoczi <stefanha@redhat.com> 
> > > > wrote:
> > > > >
> > > > > On Wed, Dec 09, 2020 at 07:41:23PM +0100, Eugenio Perez Martin wrote:
> > > > > > On Tue, Dec 8, 2020 at 9:16 AM Stefan Hajnoczi <stefanha@gmail.com> 
> > > > > > wrote:
> > > > > > > On Fri, Nov 20, 2020 at 07:50:51PM +0100, Eugenio Pérez wrote:
> > > > > > > > +        while (true) {
> > > > > > > > +            int r;
> > > > > > > > +            if (virtio_queue_full(vq)) {
> > > > > > > > +                break;
> > > > > > > > +            }
> > > > > > >
> > > > > > > Why is this check necessary? The guest cannot provide more 
> > > > > > > descriptors
> > > > > > > than there is ring space. If that happens somehow then it's a 
> > > > > > > driver
> > > > > > > error that is already reported in virtqueue_pop() below.
> > > > > > >
> > > > > >
> > > > > > It's just checked because virtqueue_pop prints an error on that 
> > > > > > case,
> > > > > > and there is no way to tell the difference between a regular error 
> > > > > > and
> > > > > > another caused by other causes. Maybe the right thing to do is just 
> > > > > > to
> > > > > > not to print that error? Caller should do the error printing in that
> > > > > > case. Should we return an error code?
> > > > >
> > > > > The reason an error is printed today is because it's a guest error 
> > > > > that
> > > > > never happens with correct guest drivers. Something is broken and the
> > > > > user should know about it.
> > > > >
> > > > > Why is "virtio_queue_full" (I already forgot what that actually means,
> > > > > it's not clear whether this is referring to avail elements or used
> > > > > elements) a condition that should be silently ignored in shadow vqs?
> > > > >
> > > >
> > > > TL;DR: It can be changed to a check of the number of available
> > > > descriptors in shadow vq, instead of returning as a regular operation.
> > > > However, I think that making it a special return of virtqueue_pop
> > > > could help in devices that run to completion, avoiding having to
> > > > duplicate the count logic in them.
> > > >
> > > > The function virtio_queue_empty checks if the vq has all descriptors
> > > > available, so the device has no more work to do until the driver makes
> > > > another descriptor available. I can see how it can be a bad name
> > > > choice, but virtio_queue_full means the opposite: device has pop()
> > > > every descriptor available, and it has not returned any, so the driver
> > > > cannot progress until the device marks some descriptors as used.
> > > >
> > > > As I understand, if vq->in_use >vq->num would mean we have a bug in
> > > > the device vq code, not in the driver. virtio_queue_full could even be
> > > > changed to "assert(vq->inuse <= vq->vring.num); return vq->inuse ==
> > > > vq->vring.num", as long as vq->in_use is operated right.
> > > >
> > > > If we hit vq->in_use == vq->num in virtqueue_pop it means the device
> > > > tried to pop() one more buffer after having all of them available and
> > > > pop'ed. This would be invalid if the device is counting right the
> > > > number of in_use descriptors, but then we are duplicating that logic
> > > > in the device and the vq.
> >
> > Devices call virtqueue_pop() until it returns NULL. They don't need to
> > count virtqueue buffers explicitly. It returns NULL when vq->num
> > virtqueue buffers have already been popped (either because
> > virtio_queue_empty() is true or because an invalid driver state is
> > detected by checking vq->num in virtqueue_pop()).
> 
> If I understood you right, the virtio_queue_full addresses the reverse
> problem: it controls when the virtqueue is out of buffers to make
> available for the device because the latter has not consumed any, not
> when the driver does not offer more buffers to the device because it
> has no more data to offer.
> 
> I find it easier to explain with the virtio-net rx queue (and I think
> it's the easier way to trigger this issue). You are describing it's
> regular behavior: The guest fills it (let's say 100%), and the device
> picks buffers one by one:
> 
> virtio_net_receive_rcu:
> while (offset < size) {
>     elem = virtqueue_pop(q->rx_vq, sizeof(VirtQueueElement));

The lines before this loop return early when the virtqueue does not have
sufficient buffer space:

  if (!virtio_net_has_buffers(q, size + n->guest_hdr_len - n->host_hdr_len)) {
      return 0;
  }

When entering this loop we know that we can pop the buffers needed to
fill one rx packet.

>     if (!elem) {
>         virtio_error("unexpected empty queue");
>     }
>     /* [1] */
>     /* fill elem with rx packet */
>     virtqueue_fill(virtqueue, elem);
>     ...
>     virtqueue_flush(q->rx_vq, i);
> }
> 
> Every device as far as I know does this buffer by buffer, there is
> just processing code in [1], and it never tries to pop more than one
> buffers/chain of buffers at the same time. In the case of a queue
> empty (no more available buffers), we hit an error, because there are
> no more buffers to write.

It's an error because we already checked that the virtqueue has buffer
space. This should never happen.

> In other devices (or tx queue), empty
> buffers means there is no more work to do, not an error.
> 
> In the case of shadow virtqueue, we cannot limit the number of exposed
> rx buffers to 1 buffer/chain of buffers in [1], since it will affect
> batching. We have the opposite problem: All devices (but rx queue)
> want to queue "as empty as possible", or "to mark all available
> buffers empty". Net rx queue is ok as long as it has a buffer/buffer
> chain big enough to write to, but it will fetch them on demand, so
> "queue full" (as in all buffers are available) is not a problem for
> the device.
> 
> However, the part of the shadow virtqueue that forwards the available
> buffer seeks the opposite: It wants as many buffers as possible to be
> available. That means that there is no [1] code that fills/read &
> flush/detach the buffer immediately: Shadow virtqueue wants to make
> available as many buffers as possible, but the device may not use them
> until it has more data available. To the extreme (virtio-net rx queue
> full), shadow virtqueue may make available all buffers, so in a
> while(true) loop, it will try to make them available until it hits
> that all the buffers are already available (vq->in_use == vq->num).
> 
> The solution is to check the number of buffers already available
> before calling virtio_queue_pop(). We could duplicate in_use in shadow
> virtqueue, of course, but everything we need is already done in
> VirtQueue code, so I think to reuse it is a better solution. Another
> solution could be to treat vq->in_use == vq->num as an special return
> code with no printed error in virtqueue_pop, but to expose if the
> queue is full (as vq->in_use == vq->num) sounds less invasive to me.
>
> >
> > > > In shadow vq this situation happens with the correct guest network
> > > > driver, since the rx queue is filled for the device to write. Network
> > > > device in qemu fetch descriptors on demand, but shadow vq fetch all
> > > > available in batching. If the driver just happens to fill the queue of
> > > > available descriptors, the log will raise, so we need to check in
> > > > handle_sw_lm_vq before calling pop(). Of course the shadow vq can
> > > > duplicate guest_vq->in_use instead of checking virtio_queue_full, but
> > > > then it needs to check two things for every virtqueue_pop() [1].
> >
> > I don't understand this scenario. It sounds like you are saying the
> > guest and shadow rx vq are not in sync so there is a case where
> > vq->in_use > vq->num is triggered?
> 
> Sorry if I explain it bad, what I meant is that there is a case where
> SVQ (as device code) will call virtqueue_pop() when vq->in_use ==
> vq->num. virtio_queue_full maintains the check as >=, I think it
> should be safe to even to code virtio_queue_full to:
> 
> assert(vq->in_use > vq->num);
> return vq->inuse == vq->num;
> 
> Please let me know if this is not clear enough.

I don't get it. When virtqueue_split_pop() has popped all requests
virtio_queue_empty_rcu() should return true and we shouldn't reach if
(vq->inuse >= vq->vring.num). The guest driver cannot submit more
available buffers at this point.

I only checked split rings, not packed rings.

Can you point to the SVQ code which has this problem? It may be easier
to re-read the code than try to describe it in an email.

Stefan

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