Re: [Qemu-devel] [PATCH v2 for-2.3] virtio-blk: correctly dirty guest memory

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

On Thu, Apr 02, 2015 at 07:50:44PM +0200, Paolo Bonzini wrote:
> After qemu_iovec_destroy, the QEMUIOVector's size is zeroed and
> the zero size ultimately is used to compute virtqueue_push's len
> argument.  Therefore, reads from virtio-blk devices did not
> migrate their results correctly.  (Writes were okay).
> 
> Save the size in virtio_blk_handle_request, and use it when the request
> is completed.
> 
> Based on a patch by Wen Congyang.
> 
> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

If you are touching this code anyway, maybe it makes
sense to merge Rusty's virtio len patches?
I didn't want them in 2.3 since they aren't ciritical,
but we are changing these lines anyway, maybe make
them correct?

> ---
>  hw/block/dataplane/virtio-blk.c |  3 +--
>  hw/block/virtio-blk.c           | 13 ++++++++++++-
>  include/hw/virtio/virtio-blk.h  |  1 +
>  3 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index cd41478..3db139b 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -77,8 +77,7 @@ static void complete_request_vring(VirtIOBlockReq *req, unsigned char status)
>      VirtIOBlockDataPlane *s = req->dev->dataplane;
>      stb_p(&req->in->status, status);
>  
> -    vring_push(s->vdev, &req->dev->dataplane->vring, &req->elem,
> -               req->qiov.size + sizeof(*req->in));
> +    vring_push(s->vdev, &req->dev->dataplane->vring, &req->elem, req->in_len);
>  
>      /* Suppress notification to guest by BH and its scheduled
>       * flag because requests are completed as a batch after io
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 000c38d..9546fd2 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -33,6 +33,7 @@ VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
>      VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);
>      req->dev = s;
>      req->qiov.size = 0;
> +    req->in_len = 0;
>      req->next = NULL;
>      req->mr_next = NULL;
>      return req;
> @@ -54,7 +55,7 @@ static void virtio_blk_complete_request(VirtIOBlockReq *req,
>      trace_virtio_blk_req_complete(req, status);
>  
>      stb_p(&req->in->status, status);
> -    virtqueue_push(s->vq, &req->elem, req->qiov.size + sizeof(*req->in));
> +    virtqueue_push(s->vq, &req->elem, req->in_len);
>      virtio_notify(vdev, s->vq);
>  }
>  
> @@ -102,6 +103,14 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
>          if (ret) {
>              int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
>              bool is_read = !(p & VIRTIO_BLK_T_OUT);
> +            /* Note that memory may be dirtied on read failure.  If the
> +             * virtio request is not completed here, as is the case for
> +             * BLOCK_ERROR_ACTION_STOP, the memory may not be copied
> +             * correctly during live migration.  While this is ugly,
> +             * it is acceptable because the device is free to write to
> +             * the memory until the request is completed (which will
> +             * happen on the other side of the migration).
> +             */
>              if (virtio_blk_handle_rw_error(req, -ret, is_read)) {
>                  continue;
>              }
> @@ -496,6 +505,8 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
>          exit(1);
>      }
>  
> +    /* We always touch the last byte, so just see how big in_iov is.  */
> +    req->in_len = iov_size(in_iov, in_num);
>      req->in = (void *)in_iov[in_num - 1].iov_base
>                + in_iov[in_num - 1].iov_len
>                - sizeof(struct virtio_blk_inhdr);
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index b3ffcd9..6bf5905 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -67,6 +67,7 @@ typedef struct VirtIOBlockReq {
>      struct virtio_blk_inhdr *in;
>      struct virtio_blk_outhdr out;
>      QEMUIOVector qiov;
> +    size_t in_len;
>      struct VirtIOBlockReq *next;
>      struct VirtIOBlockReq *mr_next;
>      BlockAcctCookie acct;
> -- 
> 2.3.4