[Qemu-devel] Re: [PATCH 05/21] virtio: modify save/load handler to handle inuse varialble.

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

On Thu, Dec 16, 2010 at 04:36:16PM +0900, Yoshiaki Tamura wrote:
> 2010/12/3 Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>:
> > 2010/12/2 Michael S. Tsirkin <mst@redhat.com>:
> >> On Wed, Dec 01, 2010 at 05:03:43PM +0900, Yoshiaki Tamura wrote:
> >>> 2010/11/28 Michael S. Tsirkin <mst@redhat.com>:
> >>> > On Sun, Nov 28, 2010 at 08:27:58PM +0900, Yoshiaki Tamura wrote:
> >>> >> 2010/11/28 Michael S. Tsirkin <mst@redhat.com>:
> >>> >> > On Thu, Nov 25, 2010 at 03:06:44PM +0900, Yoshiaki Tamura wrote:
> >>> >> >> Modify inuse type to uint16_t, let save/load to handle, and revert
> >>> >> >> last_avail_idx with inuse if there are outstanding emulation.
> >>> >> >>
> >>> >> >> Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
> >>> >> >
> >>> >> > This changes migration format, so it will break compatibility with
> >>> >> > existing drivers. More generally, I think migrating internal
> >>> >> > state that is not guest visible is always a mistake
> >>> >> > as it ties migration format to an internal implementation
> >>> >> > (yes, I know we do this sometimes, but we should at least
> >>> >> > try not to add such cases).  I think the right thing to do in this case
> >>> >> > is to flush outstanding
> >>> >> > work when vm is stopped.  Then, we are guaranteed that inuse is 0.
> >>> >> > I sent patches that do this for virtio net and block.
> >>> >>
> >>> >> Could you give me the link of your patches?  I'd like to test
> >>> >> whether they work with Kemari upon failover.  If they do, I'm
> >>> >> happy to drop this patch.
> >>> >>
> >>> >> Yoshi
> >>> >
> >>> > Look for this:
> >>> > stable migration image on a stopped vm
> >>> > sent on:
> >>> > Wed, 24 Nov 2010 17:52:49 +0200
> >>>
> >>> Thanks for the info.
> >>>
> >>> However, The patch series above didn't solve the issue.  In
> >>> case of Kemari, inuse is mostly > 0 because it queues the
> >>> output, and while last_avail_idx gets incremented
> >>> immediately, not sending inuse makes the state inconsistent
> >>> between Primary and Secondary.
> >>
> >> Hmm. Can we simply avoid incrementing last_avail_idx?
> >
> > I think we can calculate or prepare an internal last_avail_idx,
> > and update the external when inuse is decremented.  I'll try
> > whether it work w/ w/o Kemari.
> 
> Hi Michael,
> 
> Could you please take a look at the following patch?

Which version is this against?

> commit 36ee7910059e6b236fe9467a609f5b4aed866912
> Author: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
> Date:   Thu Dec 16 14:50:54 2010 +0900
> 
>     virtio: update last_avail_idx when inuse is decreased.
> 
>     Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>

It would be better to have a commit description explaining why a change
is made, and why it is correct, not just repeating what can be seen from
the diff anyway.

> diff --git a/hw/virtio.c b/hw/virtio.c
> index c8a0fc6..6688c02 100644
> --- a/hw/virtio.c
> +++ b/hw/virtio.c
> @@ -237,6 +237,7 @@ void virtqueue_flush(VirtQueue *vq, unsigned int count)
>      wmb();
>      trace_virtqueue_flush(vq, count);
>      vring_used_idx_increment(vq, count);
> +    vq->last_avail_idx += count;
>      vq->inuse -= count;
>  }
> 
> @@ -385,7 +386,7 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
>      unsigned int i, head, max;
>      target_phys_addr_t desc_pa = vq->vring.desc;
> 
> -    if (!virtqueue_num_heads(vq, vq->last_avail_idx))
> +    if (!virtqueue_num_heads(vq, vq->last_avail_idx + vq->inuse))
>          return 0;
> 
>      /* When we start there are none of either input nor output. */
> @@ -393,7 +394,7 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
> 
>      max = vq->vring.num;
> 
> -    i = head = virtqueue_get_head(vq, vq->last_avail_idx++);
> +    i = head = virtqueue_get_head(vq, vq->last_avail_idx + vq->inuse);
> 
>      if (vring_desc_flags(desc_pa, i) & VRING_DESC_F_INDIRECT) {
>          if (vring_desc_len(desc_pa, i) % sizeof(VRingDesc)) {
> 

Hmm, will virtio_queue_empty be wrong now? What about virtqueue_avail_bytes?
Previous patch version sure looked simpler, and this seems functionally
equivalent, so my question still stands: here it is rephrased in a
different way:

	assume that we have in avail ring 2 requests at start of ring: A and B in this order

	host pops A, then B, then completes B and flushes

	now with this patch last_avail_idx will be 1, and then
	remote will get it, it will execute B again. As a result
	B will complete twice, and apparently A will never complete.


This is what I was saying below: assuming that there are
outstanding requests when we migrate, there is no way
a single index can be enough to figure out which requests
need to be handled and which are in flight already.

We must add some kind of bitmask to tell us which is which.

> >
> >>
> >>>  I'm wondering why
> >>> last_avail_idx is OK to send but not inuse.
> >>
> >> last_avail_idx is at some level a mistake, it exposes part of
> >> our internal implementation, but it does *also* express
> >> a guest observable state.
> >>
> >> Here's the problem that it solves: just looking at the rings in virtio
> >> there is no way to detect that a specific request has already been
> >> completed. And the protocol forbids completing the same request twice.
> >>
> >> Our implementation always starts processing the requests
> >> in order, and since we flush outstanding requests
> >> before save, it works to just tell the remote 'process only requests
> >> after this place'.
> >>
> >> But there's no such requirement in the virtio protocol,
> >> so to be really generic we could add a bitmask of valid avail
> >> ring entries that did not complete yet. This would be
> >> the exact representation of the guest observable state.
> >> In practice we have rings of up to 512 entries.
> >> That's 64 byte per ring, not a lot at all.
> >>
> >> However, if we ever do change the protocol to send the bitmask,
> >> we would need some code to resubmit requests
> >> out of order, so it's not trivial.
> >>
> >> Another minor mistake with last_avail_idx is that it has
> >> some redundancy: the high bits in the index
> >> (> vq size) are not necessary as they can be
> >> got from avail idx.  There's a consistency check
> >> in load but we really should try to use formats
> >> that are always consistent.
> >>
> >>> The following patch does the same thing as original, yet
> >>> keeps the format of the virtio.  It shouldn't break live
> >>> migration either because inuse should be 0.
> >>>
> >>> Yoshi
> >>
> >> Question is, can you flush to make inuse 0 in kemari too?
> >> And if not, how do you handle the fact that some requests
> >> are in flight on the primary?
> >
> > Although we try flushing requests one by one making inuse 0,
> > there are cases when it failovers to the secondary when inuse
> > isn't 0.  We handle these in flight request on the primary by
> > replaying on the secondary.
> >
> >>
> >>> diff --git a/hw/virtio.c b/hw/virtio.c
> >>> index c8a0fc6..875c7ca 100644
> >>> --- a/hw/virtio.c
> >>> +++ b/hw/virtio.c
> >>> @@ -664,12 +664,16 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
> >>>      qemu_put_be32(f, i);
> >>>
> >>>      for (i = 0; i < VIRTIO_PCI_QUEUE_MAX; i++) {
> >>> +        uint16_t last_avail_idx;
> >>> +
> >>>          if (vdev->vq[i].vring.num == 0)
> >>>              break;
> >>>
> >>> +        last_avail_idx = vdev->vq[i].last_avail_idx - vdev->vq[i].inuse;
> >>> +
> >>>          qemu_put_be32(f, vdev->vq[i].vring.num);
> >>>          qemu_put_be64(f, vdev->vq[i].pa);
> >>> -        qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);
> >>> +        qemu_put_be16s(f, &last_avail_idx);
> >>>          if (vdev->binding->save_queue)
> >>>              vdev->binding->save_queue(vdev->binding_opaque, i, f);
> >>>      }
> >>>
> >>>
> >>
> >> This looks wrong to me.  Requests can complete in any order, can they
> >> not?  So if request 0 did not complete and request 1 did not,
> >> you send avail - inuse and on the secondary you will process and
> >> complete request 1 the second time, crashing the guest.
> >
> > In case of Kemari, no.  We sit between devices and net/block, and
> > queue the requests.  After completing each transaction, we flush
> > the requests one by one.  So there won't be completion inversion,
> > and therefore won't be visible to the guest.
> >
> > Yoshi
> >
> >>
> >>>
> >>> >
> >>> >> >
> >>> >> >> ---
> >>> >> >>  hw/virtio.c |    8 +++++++-
> >>> >> >>  1 files changed, 7 insertions(+), 1 deletions(-)
> >>> >> >>
> >>> >> >> diff --git a/hw/virtio.c b/hw/virtio.c
> >>> >> >> index 849a60f..5509644 100644
> >>> >> >> --- a/hw/virtio.c
> >>> >> >> +++ b/hw/virtio.c
> >>> >> >> @@ -72,7 +72,7 @@ struct VirtQueue
> >>> >> >>      VRing vring;
> >>> >> >>      target_phys_addr_t pa;
> >>> >> >>      uint16_t last_avail_idx;
> >>> >> >> -    int inuse;
> >>> >> >> +    uint16_t inuse;
> >>> >> >>      uint16_t vector;
> >>> >> >>      void (*handle_output)(VirtIODevice *vdev, VirtQueue *vq);
> >>> >> >>      VirtIODevice *vdev;
> >>> >> >> @@ -671,6 +671,7 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
> >>> >> >>          qemu_put_be32(f, vdev->vq[i].vring.num);
> >>> >> >>          qemu_put_be64(f, vdev->vq[i].pa);
> >>> >> >>          qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);
> >>> >> >> +        qemu_put_be16s(f, &vdev->vq[i].inuse);
> >>> >> >>          if (vdev->binding->save_queue)
> >>> >> >>              vdev->binding->save_queue(vdev->binding_opaque, i, f);
> >>> >> >>      }
> >>> >> >> @@ -711,6 +712,11 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
> >>> >> >>          vdev->vq[i].vring.num = qemu_get_be32(f);
> >>> >> >>          vdev->vq[i].pa = qemu_get_be64(f);
> >>> >> >>          qemu_get_be16s(f, &vdev->vq[i].last_avail_idx);
> >>> >> >> +        qemu_get_be16s(f, &vdev->vq[i].inuse);
> >>> >> >> +
> >>> >> >> +        /* revert last_avail_idx if there are outstanding emulation. */
> >>> >> >> +        vdev->vq[i].last_avail_idx -= vdev->vq[i].inuse;
> >>> >> >> +        vdev->vq[i].inuse = 0;
> >>> >> >>
> >>> >> >>          if (vdev->vq[i].pa) {
> >>> >> >>              virtqueue_init(&vdev->vq[i]);
> >>> >> >> --
> >>> >> >> 1.7.1.2
> >>> >> >>
> >>> >> >> --
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