Re: [PATCH 06/19] virtio: update last_avail_idx when inuse is decreased.

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

On Fri, Dec 24, 2010 at 08:22:00PM +0900, Yoshiaki Tamura wrote:
> 2010/12/24 Michael S. Tsirkin <mst@redhat.com>:
> > On Fri, Dec 24, 2010 at 12:18:15PM +0900, Yoshiaki Tamura wrote:
> >> virtio save/load is currently sending last_avail_idx, but inuse isn't.
> >> This causes inconsistent state when using Kemari which replays
> >> outstanding requests on the secondary.  By letting last_avail_idx to
> >> be updated after inuse is decreased, it would be possible to replay
> >> the outstanding requests.  Noth that live migration shouldn't be
> >> affected because it waits until flushing all requests.  Also in
> >> conjunction with event-tap, requests inversion should be avoided.
> >>
> >> Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
> >
> > I think I understood the request inversion. My question now is,
> > event-tap transfers inuse events as well, wont the same
> > request be repeated twice?
> >
> >> ---
> >>  hw/virtio.c |    8 +++++++-
> >>  1 files changed, 7 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/hw/virtio.c b/hw/virtio.c
> >> index 07dbf86..f915c46 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);
> >>      }
> >> @@ -710,6 +711,11 @@ 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. */
> >
> > if there are outstanding emulation -> if requests
> > are outstanding in event-tap?
> >
> >> +        vdev->vq[i].last_avail_idx -= vdev->vq[i].inuse;
> >> +        vdev->vq[i].inuse = 0;
> >>
> >
> > I don't understand it, if this is all we do we can equivalently
> > decrement on the sender side and avoid breaking migration compatibility?
> 
> It seems I sent the old patch...  I'm really sorry.  Currently
> I'm taking the approach to update last_avai_idx later.
> Decreasing looks scary to me if the guest already knows about it.

It seems exactly the same functionally.

> commit 8ac6ba51cc558b3bfcac7a5814d92f275ee874e9
> Author: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
> Date:   Mon May 17 10:36:14 2010 +0900
> 
>     virtio: update last_avail_idx when inuse is decreased.
> 
>     virtio save/load is currently sending last_avail_idx, but inuse isn't.
>     This causes inconsistent state when using Kemari which replays
>     outstanding requests on the secondary.  By letting last_avail_idx to
>     be updated after inuse is decreased, it would be possible to replay
>     the outstanding requests.  Noth that live migration shouldn't be
>     affected because it waits until flushing all requests.  Also in
>     conjunction with event-tap, requests inversion should be avoided.
> 
>     Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
> 
> diff --git a/hw/virtio.c b/hw/virtio.c
> index 07dbf86..b1586da 100644
> --- a/hw/virtio.c
> +++ b/hw/virtio.c
> @@ -198,7 +198,7 @@ int virtio_queue_ready(VirtQueue *vq)
> 
>  int virtio_queue_empty(VirtQueue *vq)
>  {
> -    return vring_avail_idx(vq) == vq->last_avail_idx;
> +    return vring_avail_idx(vq) == vq->last_avail_idx + vq->inuse;
>  }
> 
>  void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
> @@ -238,6 +238,7 @@ 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;
>  }
> 
> @@ -306,7 +307,7 @@ int virtqueue_avail_bytes(VirtQueue *vq, int in_bytes, int o
>      unsigned int idx;
>      int total_bufs, in_total, out_total;
> 
> -    idx = vq->last_avail_idx;
> +    idx = vq->last_avail_idx + vq->inuse;
> 
>      total_bufs = in_total = out_total = 0;
>      while (virtqueue_num_heads(vq, idx)) {
> @@ -386,7 +387,7 @@ 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. */
> @@ -394,7 +395,7 @@ 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)) {
> @@ -626,7 +627,7 @@ void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
>      /* Always notify when queue is empty (when feature acknowledge) */
>      if ((vring_avail_flags(vq) & VRING_AVAIL_F_NO_INTERRUPT) &&
>          (!(vdev->guest_features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY)) ||
> -         (vq->inuse || vring_avail_idx(vq) != vq->last_avail_idx)))
> +         (vq->inuse || vring_avail_idx(vq) != vq->last_avail_idx + vq->inuse)))
>          return;
> 
>      trace_virtio_notify(vdev, vq);
> 
> 
> >
> >>          if (vdev->vq[i].pa) {
> >>              uint16_t nheads;
> >> --
> >> 1.7.1.2
> > --
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