This function tries to request, if not granted yet, for the given
permissions.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/block-backend.c          | 12 ++++++++++++
 include/sysemu/block-backend.h |  1 +
 2 files changed, 13 insertions(+)

diff --git a/block/block-backend.c b/block/block-backend.c
index 5742c09..5d17404 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -595,6 +595,18 @@ void blk_get_perm(BlockBackend *blk, uint64_t *perm, uint64_t *shared_perm)
     *shared_perm = blk->shared_perm;
 }
 
+int blk_request_perm(BlockBackend *blk, uint64_t perm, Error **errp)
+{
+    uint64_t blk_perm, shared_perm;
+
+    blk_get_perm(blk, &blk_perm, &shared_perm);
+    if ((blk_perm & perm) == perm) {
+        return 0;
+    }
+    blk_perm |= perm;
+    return blk_set_perm(blk, blk_perm, shared_perm, errp);
+}
+
 static int blk_do_attach_dev(BlockBackend *blk, void *dev)
 {
     if (blk->dev) {
diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 096c17f..016632e 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -108,6 +108,7 @@ bool bdrv_is_root_node(BlockDriverState *bs);
 int blk_set_perm(BlockBackend *blk, uint64_t perm, uint64_t shared_perm,
                  Error **errp);
 void blk_get_perm(BlockBackend *blk, uint64_t *perm, uint64_t *shared_perm);
+int blk_request_perm(BlockBackend *blk, uint64_t perm, Error **errp);
 
 void blk_set_allow_write_beyond_eof(BlockBackend *blk, bool allow);
 void blk_iostatus_enable(BlockBackend *blk);
-- 
2.9.3