scsi: support NDOB (no data-out buffer) for WRITE SAME commands

A NDOB bit set to one specifies that the disk shall not transfer data
from the data-out buffer and shall process the command as if the data-out
buffer contained user data set to all zeroes.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-bus.c

@@ -944,7 +944,7 @@ static int scsi_req_xfer(SCSICommand *cmd, SCSIDevice *dev, uint8_t *buf)
         break;
     case WRITE_SAME_10:
     case WRITE_SAME_16:
-        cmd->xfer = dev->blocksize;
+        cmd->xfer = buf[1] & 1 ? 0 : dev->blocksize;
         break;
     case READ_CAPACITY_10:
         cmd->xfer = 8;

hw/scsi/scsi-disk.c

@@ -1807,7 +1807,7 @@ static void scsi_disk_emulate_write_same(SCSIDiskReq *r, uint8_t *inbuf)
         return;
     }
 
-    if (buffer_is_zero(inbuf, s->qdev.blocksize)) {
+    if ((req->cmd.buf[1] & 0x1) || buffer_is_zero(inbuf, s->qdev.blocksize)) {
         int flags = (req->cmd.buf[1] & 0x8) ? BDRV_REQ_MAY_UNMAP : 0;
 
         /* The request is used as the AIO opaque value, so add a ref.  */

tests/virtio-scsi-test.c

@@ -216,6 +216,9 @@ static void test_unaligned_write_same(void)
     const uint8_t write_same_cdb_2[VIRTIO_SCSI_CDB_SIZE] = {
         0x41, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x33, 0x00, 0x00
     };
+    const uint8_t write_same_cdb_ndob[VIRTIO_SCSI_CDB_SIZE] = {
+        0x41, 0x01, 0x00, 0x00, 0x00, 0x01, 0x00, 0x33, 0x00, 0x00
+    };
 
     vs = qvirtio_scsi_pci_init(PCI_SLOT);
 
@@ -225,6 +228,9 @@ static void test_unaligned_write_same(void)
     g_assert_cmphex(0, ==,
         virtio_scsi_do_command(vs, write_same_cdb_2, NULL, 0, buf2, 512, NULL));
 
+    g_assert_cmphex(0, ==,
+        virtio_scsi_do_command(vs, write_same_cdb_ndob, NULL, 0, NULL, 0, NULL));
+
     qvirtio_scsi_pci_free(vs);
 }