block: Give nonzero result to blk_get_max_transfer_length()

Making all callers special-case 0 as unlimited is awkward,
and we DO have a hard maximum of BDRV_REQUEST_MAX_SECTORS given
our current block layer API limits.

In the case of scsi, this means that we now always advertise a
limit to the guest, even in cases where the underlying layers
previously use 0 for no inherent limit beyond the block layer.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-backend.c

@@ -1303,15 +1303,16 @@ int blk_get_flags(BlockBackend *blk)
     }
 }
 
+/* Returns the maximum transfer length, in sectors; guaranteed nonzero */
 int blk_get_max_transfer_length(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
+    int max = 0;
 
     if (bs) {
-        return bs->bl.max_transfer_length;
-    } else {
-        return 0;
+        max = bs->bl.max_transfer_length;
     }
+    return MIN_NON_ZERO(max, BDRV_REQUEST_MAX_SECTORS);
 }
 
 int blk_get_max_iov(BlockBackend *blk)

hw/block/virtio-blk.c

@@ -384,7 +384,7 @@ static int multireq_compare(const void *a, const void *b)
 void virtio_blk_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb)
 {
     int i = 0, start = 0, num_reqs = 0, niov = 0, nb_sectors = 0;
-    int max_xfer_len = 0;
+    int max_xfer_len;
     int64_t sector_num = 0;
 
     if (mrb->num_reqs == 1) {
@@ -394,7 +394,6 @@ void virtio_blk_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb)
     }
 
     max_xfer_len = blk_get_max_transfer_length(mrb->reqs[0]->dev->blk);
-    max_xfer_len = MIN_NON_ZERO(max_xfer_len, BDRV_REQUEST_MAX_SECTORS);
 
     qsort(mrb->reqs, mrb->num_reqs, sizeof(*mrb->reqs),
           &multireq_compare);

hw/scsi/scsi-generic.c

@@ -227,12 +227,12 @@ static void scsi_read_complete(void * opaque, int ret)
         r->req.cmd.buf[2] == 0xb0) {
         uint32_t max_xfer_len = blk_get_max_transfer_length(s->conf.blk) /
             (s->blocksize / BDRV_SECTOR_SIZE);
-        if (max_xfer_len) {
-            stl_be_p(&r->buf[8], max_xfer_len);
-            /* Also take care of the opt xfer len. */
-            if (ldl_be_p(&r->buf[12]) > max_xfer_len) {
-                stl_be_p(&r->buf[12], max_xfer_len);
-            }
+
+        assert(max_xfer_len);
+        stl_be_p(&r->buf[8], max_xfer_len);
+        /* Also take care of the opt xfer len. */
+        if (ldl_be_p(&r->buf[12]) > max_xfer_len) {
+            stl_be_p(&r->buf[12], max_xfer_len);
         }
     }
     scsi_req_data(&r->req, len);