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

[Fam Zheng]

On Thu, 04/02 17:21, Paolo Bonzini wrote:
> 
> 
> On 02/04/2015 17:16, Fam Zheng 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).
> >>> > > 
> >>> > > Can't we move qemu_iovec_destroy to virtio_blk_free_request?
> >> > 
> >> > You would still have to add more code to differentiate reads and
> >> > writes---I think.
> > Yeah, but the extra field will not be needed.
> 
> Can you post an alternative patch?  One small complication is that
> is_write is in mrb but not in mrb->reqs[x].  virtio_blk_rw_complete is
> already doing
> 
>     int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
>     bool is_read = !(p & VIRTIO_BLK_T_OUT);
> 
> but only in a slow path.

OK, so it looks like a new field is the simplest way to achieve.

There is another problem with your patch - read_size is not initialized in
non-RW paths like scsi and flush.

I think the optimization for write is a separate thing, though. Shouldn't below
patch already fix the migration issue?

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 000c38d..ee6e198 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -92,13 +92,6 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
         next = req->mr_next;
         trace_virtio_blk_rw_complete(req, ret);
 
-        if (req->qiov.nalloc != -1) {
-            /* If nalloc is != 1 req->qiov is a local copy of the original
-             * external iovec. It was allocated in submit_merged_requests
-             * to be able to merge requests. */
-            qemu_iovec_destroy(&req->qiov);
-        }
-
         if (ret) {
             int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
             bool is_read = !(p & VIRTIO_BLK_T_OUT);
@@ -109,6 +102,13 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
 
         virtio_blk_req_complete(req, VIRTIO_BLK_S_OK);
         block_acct_done(blk_get_stats(req->dev->blk), &req->acct);
+
+        if (req->qiov.nalloc != -1) {
+            /* This means req->qiov is a local copy of the original external
+             * iovec. It was allocated in virtio_blk_submit_multireq in order
+             * to merge requests. */
+            qemu_iovec_destroy(&req->qiov);
+        }
         virtio_blk_free_request(req);
     }
 }