[SLOF] [PATCH] virtio-scsi: Fix descriptor order for SCSI WRITE commands

[Thomas Huth]

Virtio needs the descriptors ordered: All descriptors for buffers that
contain data that should go *to* the device have to be put into the
queue first. The descriptors for buffers that read *from* the device
have to be sorted in last. For SCSI WRITE commands (which we never
used in the past), the order was wrong, so QEMU complains with a
"Incorrect order for descriptors" message as soon as we use it for
SCSI WRITE commands.

Signed-off-by: Thomas Huth <thuth at redhat.com>
---
 lib/libvirtio/virtio-scsi.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/lib/libvirtio/virtio-scsi.c b/lib/libvirtio/virtio-scsi.c
index 04181b0..36b62d1 100644
--- a/lib/libvirtio/virtio-scsi.c
+++ b/lib/libvirtio/virtio-scsi.c
@@ -49,18 +49,27 @@ int virtioscsi_send(struct virtio_device *dev,
 	virtio_fill_desc(&vq_desc[id], dev->is_modern, (uint64_t)req, sizeof(*req), VRING_DESC_F_NEXT,
 			 (id + 1) % vq_size);
 
-	/* Set up virtqueue descriptor for data */
-	if (buf && buf_len) {
+	if (buf == NULL || buf_len == 0) {
+		/* Set up descriptor for response information */
+		virtio_fill_desc(&vq_desc[(id + 1) % vq_size], dev->is_modern,
+				 (uint64_t)resp, sizeof(*resp),
+				 VRING_DESC_F_WRITE, 0);
+	} else if (is_read) {
+		/* Set up descriptor for response information */
 		virtio_fill_desc(&vq_desc[(id + 1) % vq_size], dev->is_modern,
 				 (uint64_t)resp, sizeof(*resp),
 				 VRING_DESC_F_NEXT | VRING_DESC_F_WRITE,
 				 (id + 2) % vq_size);
-		/* Set up virtqueue descriptor for status */
+		/* Set up virtqueue descriptor for data from device */
 		virtio_fill_desc(&vq_desc[(id + 2) % vq_size], dev->is_modern,
-				 (uint64_t)buf, buf_len,
-				 (is_read ? VRING_DESC_F_WRITE : 0), 0);
+				 (uint64_t)buf, buf_len, VRING_DESC_F_WRITE, 0);
 	} else {
+		/* Set up virtqueue descriptor for data to device */
 		virtio_fill_desc(&vq_desc[(id + 1) % vq_size], dev->is_modern,
+				 (uint64_t)buf, buf_len, VRING_DESC_F_NEXT,
+				 (id + 2) % vq_size);
+		/* Set up descriptor for response information */
+		virtio_fill_desc(&vq_desc[(id + 2) % vq_size], dev->is_modern,
 				 (uint64_t)resp, sizeof(*resp),
 				 VRING_DESC_F_WRITE, 0);
 	}
-- 
1.8.3.1