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

[Yoshiaki Tamura]

2010/12/24 Michael S. Tsirkin <address@hidden>:
> On Fri, Dec 24, 2010 at 08:22:00PM +0900, Yoshiaki Tamura wrote:
>> 2010/12/24 Michael S. Tsirkin <address@hidden>:
>> > 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 <address@hidden>
>> >
>> > 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.

If it is the same I'm fine to go with the decreasing approach.
Is it fine for the guest?  Is last_avai_idx irrelevant to the
guest's behavior?

Yoshi

>> commit 8ac6ba51cc558b3bfcac7a5814d92f275ee874e9
>> Author: Yoshiaki Tamura <address@hidden>
>> 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 <address@hidden>
>>
>> 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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