[PATCH v3] block/backup-top: fix flags handling

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

backup-top "supports" write-unchanged, by skipping CBW operation in
backup_top_co_pwritev. But it forgets to do the same in
backup_top_co_pwrite_zeroes, as well as declare support for
BDRV_REQ_WRITE_UNCHANGED.

Fix this, and, while being here, declare also support for flags
supported by source child.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---

v3: rebase on master, keep state initialization after check top != NULL.

v2: restrict flags propagation like it is done in other filters [Eric]
    move state variable initialization to the top
 block/backup-top.c | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/block/backup-top.c b/block/backup-top.c
index fa78f3256d..1bfb360bd3 100644
--- a/block/backup-top.c
+++ b/block/backup-top.c
@@ -48,11 +48,17 @@ static coroutine_fn int backup_top_co_preadv(
 }
 
 static coroutine_fn int backup_top_cbw(BlockDriverState *bs, uint64_t offset,
-                                       uint64_t bytes)
+                                       uint64_t bytes, BdrvRequestFlags flags)
 {
     BDRVBackupTopState *s = bs->opaque;
-    uint64_t end = QEMU_ALIGN_UP(offset + bytes, s->bcs->cluster_size);
-    uint64_t off = QEMU_ALIGN_DOWN(offset, s->bcs->cluster_size);
+    uint64_t off, end;
+
+    if (flags & BDRV_REQ_WRITE_UNCHANGED) {
+        return 0;
+    }
+
+    off = QEMU_ALIGN_DOWN(offset, s->bcs->cluster_size);
+    end = QEMU_ALIGN_UP(offset + bytes, s->bcs->cluster_size);
 
     return block_copy(s->bcs, off, end - off, NULL);
 }
@@ -60,7 +66,7 @@ static coroutine_fn int backup_top_cbw(BlockDriverState *bs, uint64_t offset,
 static int coroutine_fn backup_top_co_pdiscard(BlockDriverState *bs,
                                                int64_t offset, int bytes)
 {
-    int ret = backup_top_cbw(bs, offset, bytes);
+    int ret = backup_top_cbw(bs, offset, bytes, 0);
     if (ret < 0) {
         return ret;
     }
@@ -71,7 +77,7 @@ static int coroutine_fn backup_top_co_pdiscard(BlockDriverState *bs,
 static int coroutine_fn backup_top_co_pwrite_zeroes(BlockDriverState *bs,
         int64_t offset, int bytes, BdrvRequestFlags flags)
 {
-    int ret = backup_top_cbw(bs, offset, bytes);
+    int ret = backup_top_cbw(bs, offset, bytes, flags);
     if (ret < 0) {
         return ret;
     }
@@ -84,11 +90,9 @@ static coroutine_fn int backup_top_co_pwritev(BlockDriverState *bs,
                                               uint64_t bytes,
                                               QEMUIOVector *qiov, int flags)
 {
-    if (!(flags & BDRV_REQ_WRITE_UNCHANGED)) {
-        int ret = backup_top_cbw(bs, offset, bytes);
-        if (ret < 0) {
-            return ret;
-        }
+    int ret = backup_top_cbw(bs, offset, bytes, flags);
+    if (ret < 0) {
+        return ret;
     }
 
     return bdrv_co_pwritev(bs->backing, offset, bytes, qiov, flags);
@@ -196,8 +200,13 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
         return NULL;
     }
 
-    top->total_sectors = source->total_sectors;
     state = top->opaque;
+    top->total_sectors = source->total_sectors;
+    top->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
+            (BDRV_REQ_FUA & source->supported_write_flags);
+    top->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
+            ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
+             source->supported_zero_flags);
 
     bdrv_ref(target);
     state->target = bdrv_attach_child(top, target, "target", &child_file, errp);
-- 
2.21.0