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

[Michael S. Tsirkin]

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 <address@hidden>

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.

> ---
>  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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