[Qemu-block] [PATCH 3/9] block: Add BDRV_REQ_WRITE_UNCHANGED flag

[Max Reitz]

This flag signifies that a write request will not change the visible
disk content.  With this flag set, it is sufficient to have the
BLK_PERM_WRITE_UNCHANGED permission instead of BLK_PERM_WRITE.

Signed-off-by: Max Reitz <address@hidden>
---
 include/block/block.h | 6 +++++-
 block/io.c            | 6 +++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index 397b5e8d44..3894edda9d 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -54,8 +54,12 @@ typedef enum {
     BDRV_REQ_FUA                = 0x10,
     BDRV_REQ_WRITE_COMPRESSED   = 0x20,
 
+    /* Signifies that this write request will not change the visible disk
+     * content. */
+    BDRV_REQ_WRITE_UNCHANGED    = 0x40,
+
     /* Mask of valid flags */
-    BDRV_REQ_MASK               = 0x3f,
+    BDRV_REQ_MASK               = 0x7f,
 } BdrvRequestFlags;
 
 typedef struct BlockSizes {
diff --git a/block/io.c b/block/io.c
index bd9a19a9c4..134b2a498f 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1501,7 +1501,11 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild 
*child,
     assert(!waited || !req->serialising);
     assert(req->overlap_offset <= offset);
     assert(offset + bytes <= req->overlap_offset + req->overlap_bytes);
-    assert(child->perm & BLK_PERM_WRITE);
+    if (flags & BDRV_REQ_WRITE_UNCHANGED) {
+        assert(child->perm & (BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE));
+    } else {
+        assert(child->perm & BLK_PERM_WRITE);
+    }
     assert(end_sector <= bs->total_sectors || child->perm & BLK_PERM_RESIZE);
 
     ret = notifier_with_return_list_notify(&bs->before_write_notifiers, req);
-- 
2.14.3