virtio-scsi: work around bug in old BIOSes

Old BIOSes left some padding by mistake after the req_size/resp_size.
New QEMU does not like it, thinking it is a bidirectional command.

As a workaround, we can check if the ANY_LAYOUT bit is set; if not, we
always consider the first buffer as the virtio-scsi request/response,
because, back when QEMU did not support ANY_LAYOUT, it expected the
payload to start at the second element of the iovec.

This can show up during migration.

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 55783a5)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/scsi/virtio-scsi.c

@@ -135,6 +135,7 @@ static size_t qemu_sgl_concat(VirtIOSCSIReq *req, struct iovec *iov,
 static int virtio_scsi_parse_req(VirtIOSCSIReq *req,
                                  unsigned req_size, unsigned resp_size)
 {
+    VirtIODevice *vdev = (VirtIODevice *) req->dev;
     size_t in_size, out_size;
 
     if (iov_to_buf(req->elem.out_sg, req->elem.out_num, 0,
@@ -147,8 +148,24 @@ static int virtio_scsi_parse_req(VirtIOSCSIReq *req,
                               resp_size) < resp_size) {
         return -EINVAL;
     }
+
     req->resp_size = resp_size;
 
+    /* Old BIOSes left some padding by mistake after the req_size/resp_size.
+     * As a workaround, always consider the first buffer as the virtio-scsi
+     * request/response, making the payload start at the second element
+     * of the iovec.
+     *
+     * The actual length of the response header, stored in req->resp_size,
+     * does not change.
+     *
+     * TODO: always disable this workaround for virtio 1.0 devices.
+     */
+    if ((vdev->guest_features & VIRTIO_F_ANY_LAYOUT) == 0) {
+        req_size = req->elem.out_sg[0].iov_len;
+        resp_size = req->elem.in_sg[0].iov_len;
+    }
+
     out_size = qemu_sgl_concat(req, req->elem.out_sg,
                                &req->elem.out_addr[0], req->elem.out_num,
                                req_size);