[Qemu-block] [PATCH v4 3/8] block: Add blk_get_opt_transfer()

[Eric Blake]

The NBD protocol would like to advertise the optimal I/O
size to the client; but it would be a layering violation to
peek into blk_bs(blk)->bl, when we only have a BB.

This copies the existing blk_get_max_transfer() in reading
a value from the top BDS; where that value was picked via
bdrv_refresh_limits() to reflect the overall constraints of
the entire BDS chain.

Signed-off-by: Eric Blake <address@hidden>

---
v4: retitle, as part of rebasing to byte limits
v3: new patch
---
 include/sysemu/block-backend.h |  1 +
 block/block-backend.c          | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 6444e41..882480a 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -174,6 +174,7 @@ void blk_lock_medium(BlockBackend *blk, bool locked);
 void blk_eject(BlockBackend *blk, bool eject_flag);
 int blk_get_flags(BlockBackend *blk);
 uint32_t blk_get_max_transfer(BlockBackend *blk);
+uint32_t blk_get_opt_transfer(BlockBackend *blk);
 int blk_get_max_iov(BlockBackend *blk);
 void blk_set_guest_block_size(BlockBackend *blk, int align);
 void *blk_try_blockalign(BlockBackend *blk, size_t size);
diff --git a/block/block-backend.c b/block/block-backend.c
index efbf398..4d91ff8 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1426,6 +1426,18 @@ uint32_t blk_get_max_transfer(BlockBackend *blk)
     return MIN_NON_ZERO(max, INT_MAX);
 }

+/* Returns the optimum transfer length, in bytes; may be 0 if no optimum */
+uint32_t blk_get_opt_transfer(BlockBackend *blk)
+{
+    BlockDriverState *bs = blk_bs(blk);
+
+    if (bs) {
+        return bs->bl.opt_transfer;
+    } else {
+        return 0;
+    }
+}
+
 int blk_get_max_iov(BlockBackend *blk)
 {
     return blk->root->bs->bl.max_iov;
-- 
2.9.3