qapi: blockdev-backup: add discard-source parameter

Add a parameter that enables discard-after-copy. That is mostly useful
in "push backup with fleecing" scheme, when source is snapshot-access
format driver node, based on copy-before-write filter snapshot-access
API:

[guest]      [snapshot-access] ~~ blockdev-backup ~~> [backup target]
   |            |
   | root       | file
   v            v
[copy-before-write]
   |             |
   | file        | target
   v             v
[active disk]   [temp.img]

In this case discard-after-copy does two things:

 - discard data in temp.img to save disk space
 - avoid further copy-before-write operation in discarded area

Note that we have to declare WRITE permission on source in
copy-before-write filter, for discard to work. Still we can't take it
unconditionally, as it will break normal backup from RO source. So, we
have to add a parameter and pass it thorough bdrv_open flags.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20240313152822.626493-5-vsementsov@yandex-team.ru>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

3 files changed, 4 insertions, 1 deletions

diff --git a/include/block/block-common.h b/include/block/block-common.h
index a846023a09..338fe5ff7a 100644
--- a/include/block/block-common.h
+++ b/include/block/block-common.h
@@ -243,6 +243,8 @@ typedef enum {
                                       read-write fails */
 #define BDRV_O_IO_URING    0x40000 /* use io_uring instead of the thread pool */
 
+#define BDRV_O_CBW_DISCARD_SOURCE 0x80000 /* for copy-before-write filter */
+
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_NO_FLUSH)
 
 
diff --git a/include/block/block-copy.h b/include/block/block-copy.h
index 8b41643bfa..bdc703bacd 100644
--- a/include/block/block-copy.h
+++ b/include/block/block-copy.h
@@ -27,6 +27,7 @@ typedef struct BlockCopyCallState BlockCopyCallState;
 BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
                                      BlockDriverState *copy_bitmap_bs,
                                      const BdrvDirtyBitmap *bitmap,
+                                     bool discard_source,
                                      Error **errp);
 
 /* Function should be called prior any actual copy request */
diff --git a/include/block/block_int-global-state.h b/include/block/block_int-global-state.h
index d2201e27f4..eb2d92a226 100644
--- a/include/block/block_int-global-state.h
+++ b/include/block/block_int-global-state.h
@@ -193,7 +193,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                             MirrorSyncMode sync_mode,
                             BdrvDirtyBitmap *sync_bitmap,
                             BitmapSyncMode bitmap_mode,
-                            bool compress,
+                            bool compress, bool discard_source,
                             const char *filter_node_name,
                             BackupPerf *perf,
                             BlockdevOnError on_source_error,