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

Yoshiaki Tamura
2010-12-01T08:03:44Z

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. I'm wondering why
last_avail_idx is OK to send but not inuse.

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

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);
}

>
>> >
>> >> ---
>> >>  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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