Re: [PATCH v3 2/2] virtio: introduce STOP status bit

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Nov 24, 2021 at 05:41:27PM +0100, Eugenio Perez Martin wrote:
> On Wed, Nov 24, 2021 at 12:21 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > On Tue, Nov 23, 2021 at 06:00:20PM +0100, Eugenio Perez Martin wrote:
> > > On Tue, Nov 23, 2021 at 1:17 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > >
> > > > On Thu, Nov 18, 2021 at 08:58:05PM +0100, Eugenio Perez Martin wrote:
> > > > > On Thu, Nov 18, 2021 at 5:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > > > >
> > > > > > On Thu, Nov 11, 2021 at 07:58:12PM +0100, Eugenio Pérez wrote:
> > > > > > > +the driver MAY change avail_idx in the case of split virtqueue, but the new
> > > > > > > +avail_idx MUST be within used_idx and used_idx plus virtqueue size.
> > > > > >
> > > > > > I'm trying to understand how this would work. Available buffers may be
> > > > > > consumed out-of-order unless VIRTIO_F_IN_ORDER was negotiated, so the
> > > > > > avail ring could contain something like:
> > > > > >
> > > > > >   avail.ring = [Used, Not used, Used, Not used, ...]
> > > > > >                                                 ^--- avail.idx
> > > > > >
> > > > > > There are num_not_used = avail.idx - used.idx requests that are "Not
> > > > > > used" in avail.ring.
> > > > > >
> > > > > > Does this mean the driver can rewind avail.idx by counting the number of
> > > > > > "Not used" buffers and skipping "Used" buffers until it reaches
> > > > > > num_not_used "Not used" buffers?
> > > > > >
> > > > >
> > > > > I'm going to also drop the "resume" part for the next version because
> > > > > it adds extra complexity not actually needed, and it can be achieved
> > > > > with a full reset in a simpler way.
> > > > >
> > > > > But I'll explain it below with your examples. Long story short, the
> > > > > driver only can rewind the available descriptors that are still in the
> > > > > available ring, and the device must flush the ones that cannot recover
> > > > > from the ring.
> > > > >
> > > > > > I think there is a known issue with this approach:
> > > > > >
> > > > > > Imagine a vring with 4 elements:
> > > > > >
> > > > > >   avail.ring = [0,        1,    2,    3   ]
> > > > > >                 Not used, used, used, used
> > > > > >                                            ^--- avail.idx
> > > > > >
> > > > > > Since the device has used 3 buffers the driver now has space to make
> > > > > > more buffers available. avail.idx wraps back to the start of the ring
> > > > > > and the driver overwrites the first element ("Not used"):
> > > > > >
> > > > > >   avail.ring = [1,        N/A,  N/A,  N/A]
> > > > > >                 Not used, N/A,  N/A,  N/A
> > > > > >                          ^--- avail.idx
> > > > > >
> > > > > > Since vring descriptor 0 is still in use the driver chose descriptor 1
> > > > > > for the new available buffer.
> > > > > >
> > > > > > Now we stop the device, knowing there are two buffers available that
> > > > > > have not been used. But avail.ring[] actually only contains the new
> > > > > > buffer (vring descriptor 1) that we made available because we overwrote
> > > > > > the old avail.ring[] element (vring descriptor 0).
> > > > > >
> > > > > > What now? Where does the device reset its internal avail_idx to?
> > > > >
> > > > > To be on the same page, in qemu the device maintains two "internal
> > > > > avail idx": shadow_avail_idx (last seen in the available ring, could
> > > > > be 4 in this case) and last_avail_idx (next descriptor to fetch from
> > > > > avail, 2). The device must forget shadow_avail_idx and flush the
> > > > > descriptors that cannot recover (0). So last_avail_idx is now 3. Now
> > > > > it can stop.
> > > > >
> > > > > The proposal allows the device to fail descriptor 0 in a
> > > > > device-specific way, but I think now it was a bad choice.
> > > > >
> > > > > The driver cannot move the device's last_avail_idx in this operation:
> > > > > The device is simply forced to flush used ones to the used ring or
> > > > > descriptor ring in a packed vq case. So the device's internal
> > > > > avail_idx == used_idx == 3. When the device resumes, it's still 3.
> > > > >
> > > > > The device must keep its last_avail_idx through stop and resume cycle.
> > > >
> > > > Are you saying that all buffers avail->ring[i % ring_size] must be
> > > > completed by the device before the STOP bit is reported where i <=
> > > > last_avail_idx?
> > > >
> > > > This means the driver can modify avail->ring[i % ring_size] where
> > > > avail_idx >= i > used_idx.
> > > >
> > >
> > > Yes, That's correct. The driver could also decide to modify the
> > > descriptor table instead of the avail ring to do so, but I think the
> > > point is clear now.
> > >
> > > Somehow it is thought after the premise that the out of order
> >
> > "Somehow it is thought after the premise" == "there is a fundamental
> > design assumption"?
> >
> 
> Well, there always are design assumptions :). I didn't see it as
> fundamental at the time of sending it, when I didn't consider
> "idempotents in-flight descriptors" as something I could take for
> granted. So I thought of it as the best we could do with the backend
> that must wait for them, and without introducing other complicated
> things (in-flight).
> 
> > > descriptors are descriptors that the device must wait to complete
> > > before the pause anyway. Depending on the device, it might prefer to
> > > cancel them, to wait for them, etc. The interesting descriptors to
> > > rewind are the ones that have not reached the device (i > used_idx).
> > > The driver can do whatever it wants with them.
> > >
> > > If we assume all the in-flight descriptors are idempotent and we
> > > expose a way for the device to expose them, the model is way more
> > > simpler than this.
> >
> > The constraint that the device has to mark all previously seen "avail"
> > buffers as "used" is problematic. It makes STOP visible to the driver
> > when the device has to fail requests. That is incompatible with how
> > devices behave across live migration today. If you want to use STOP for
> > live migration then it's probably necessary to rethink this constraint.
> >
> > QEMU's virtio-blk and virtio-scsi device models put failed requests onto
> > a list so they can be retried after problems with the underlying storage
> > have been resolved (e.g. more disk space becomes available and ENOSPC
> > requests can be retried). Based on the constraints you described, those
> > requests cannot be kept in the list across STOP.
> >
> 
> I didn't know about that, thanks for the information! Can vhost ones
> do the same?

No, there is no vhost-blk upstream and vhost-scsi doesn't migrate failed
requests.

> > QEMU live migration sends the retry list to the migration destination. I
> > think you're saying that won't be possible when STOP is used to
> > implement live migration?
> >
> 
> Without out of order descriptor usage, the device can simply not mark
> them as used, and the destination will re-try them. Would that work?

The failed requests feature is not useful with VIRTIO_F_IN_ORDER since a
failed requests prevents further I/O processing.

> In the case of out or order, this proposal does not cover it 100% but
> the next one will do.

Great, looking forward to the next revision.

Stefan

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