virtio-blk: fix minimum number of S/G elements

We need at least one S/G element to operate properly, as does the block
layer which increments it to one anyway.  We hit this due to a qemu
bug which advertises a sg_elements of 0 under some circumstances.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (tweaked logic)

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 83fa09a836ca..258bc2ae2885 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -298,7 +298,9 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
         err = virtio_config_val(vdev, VIRTIO_BLK_F_SEG_MAX,
                                 offsetof(struct virtio_blk_config, seg_max),
                                 &sg_elems);
-        if (err)
+
+        /* We need at least one SG element, whatever they say. */
+        if (err || !sg_elems)
                 sg_elems = 1;
 
         /* We need an extra sg elements at head and tail. */