[v4,10/18] block/io: introduce block driver snapshot-access API

Commit Message

Vladimir Sementsov-Ogievskiy Feb. 16, 2022, 7:46 p.m. UTC

Add new block driver handlers and corresponding generic wrappers.
It will be used to allow copy-before-write filter to provide
reach fleecing interface in further commit.

In future this approach may be used to allow reading qcow2 interanal
snaphots, for example to export them through NBD.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[e-mail]>
---
 include/block/block_int.h | 27 +++++++++++++++
 block/io.c                | 69 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 96 insertions(+)

Comments

Hanna Czenczek Feb. 24, 2022, 12:24 p.m. UTC | #1

On 16.02.22 20:46, Vladimir Sementsov-Ogievskiy wrote:
> Add new block driver handlers and corresponding generic wrappers.
> It will be used to allow copy-before-write filter to provide
> reach fleecing interface in further commit.
>
> In future this approach may be used to allow reading qcow2 interanal

(s/interanal/internal/)

> snaphots, for example to export them through NBD.

Ooh, that’s indeed quite nice.

Raises the question of how users are to select a specific snapshot in 
qcow2 file, but your next patch answers that question: The snapshot 
access driver is to receive a runtime option for this, and the API is to 
be extended to allow for selecting a specific snapshot.  Sounds good!

> Signed-off-by: Vladimir Sementsov-Ogievskiy <[e-mail]>
> ---
>   include/block/block_int.h | 27 +++++++++++++++
>   block/io.c                | 69 +++++++++++++++++++++++++++++++++++++++
>   2 files changed, 96 insertions(+)

Yes, really nice.  Thanks.

Reviewed-by: Hanna Reitz <[e-mail]>

Patch

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 27008cfb22..c43315ae6e 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -376,6 +376,24 @@  struct BlockDriver {
      */
     void (*bdrv_cancel_in_flight)(BlockDriverState *bs);
 
+    /*
+     * Snapshot-access API.
+     *
+     * Block-driver may provide snapshot-access API: special functions to access
+     * some internal "snapshot". The functions are similar with normal
+     * read/block_status/discard handler, but don't have any specific handling
+     * in generic block-layer: no serializing, no alignment, no tracked
+     * requests. So, block-driver that realizes these APIs is fully responsible
+     * for synchronization between snapshot-access API and normal IO requests.
+     */
+    int coroutine_fn (*bdrv_co_preadv_snapshot)(BlockDriverState *bs,
+        int64_t offset, int64_t bytes, QEMUIOVector *qiov, size_t qiov_offset);
+    int coroutine_fn (*bdrv_co_snapshot_block_status)(BlockDriverState *bs,
+        bool want_zero, int64_t offset, int64_t bytes, int64_t *pnum,
+        int64_t *map, BlockDriverState **file);
+    int coroutine_fn (*bdrv_co_pdiscard_snapshot)(BlockDriverState *bs,
+        int64_t offset, int64_t bytes);
+
     /*
      * Invalidate any cached meta-data.
      */
@@ -1078,6 +1096,15 @@  extern BlockDriver bdrv_file;
 extern BlockDriver bdrv_raw;
 extern BlockDriver bdrv_qcow2;
 
+int coroutine_fn bdrv_co_preadv_snapshot(BdrvChild *child,
+    int64_t offset, int64_t bytes, QEMUIOVector *qiov, size_t qiov_offset);
+int coroutine_fn bdrv_co_snapshot_block_status(BlockDriverState *bs,
+    bool want_zero, int64_t offset, int64_t bytes, int64_t *pnum,
+    int64_t *map, BlockDriverState **file);
+int coroutine_fn bdrv_co_pdiscard_snapshot(BlockDriverState *bs,
+    int64_t offset, int64_t bytes);
+
+
 int coroutine_fn bdrv_co_preadv(BdrvChild *child,
     int64_t offset, int64_t bytes, QEMUIOVector *qiov,
     BdrvRequestFlags flags);
diff --git a/block/io.c b/block/io.c
index 4e4cb556c5..0bcf09a491 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3587,3 +3587,72 @@  void bdrv_cancel_in_flight(BlockDriverState *bs)
         bs->drv->bdrv_cancel_in_flight(bs);
     }
 }
+
+int coroutine_fn
+bdrv_co_preadv_snapshot(BdrvChild *child, int64_t offset, int64_t bytes,
+                        QEMUIOVector *qiov, size_t qiov_offset)
+{
+    BlockDriverState *bs = child->bs;
+    BlockDriver *drv = bs->drv;
+    int ret;
+
+    if (!drv) {
+        return -ENOMEDIUM;
+    }
+
+    if (!drv->bdrv_co_preadv_snapshot) {
+        return -ENOTSUP;
+    }
+
+    bdrv_inc_in_flight(bs);
+    ret = drv->bdrv_co_preadv_snapshot(bs, offset, bytes, qiov, qiov_offset);
+    bdrv_dec_in_flight(bs);
+
+    return ret;
+}
+
+int coroutine_fn
+bdrv_co_snapshot_block_status(BlockDriverState *bs,
+                              bool want_zero, int64_t offset, int64_t bytes,
+                              int64_t *pnum, int64_t *map,
+                              BlockDriverState **file)
+{
+    BlockDriver *drv = bs->drv;
+    int ret;
+
+    if (!drv) {
+        return -ENOMEDIUM;
+    }
+
+    if (!drv->bdrv_co_snapshot_block_status) {
+        return -ENOTSUP;
+    }
+
+    bdrv_inc_in_flight(bs);
+    ret = drv->bdrv_co_snapshot_block_status(bs, want_zero, offset, bytes,
+                                             pnum, map, file);
+    bdrv_dec_in_flight(bs);
+
+    return ret;
+}
+
+int coroutine_fn
+bdrv_co_pdiscard_snapshot(BlockDriverState *bs, int64_t offset, int64_t bytes)
+{
+    BlockDriver *drv = bs->drv;
+    int ret;
+
+    if (!drv) {
+        return -ENOMEDIUM;
+    }
+
+    if (!drv->bdrv_co_pdiscard_snapshot) {
+        return -ENOTSUP;
+    }
+
+    bdrv_inc_in_flight(bs);
+    ret = drv->bdrv_co_pdiscard_snapshot(bs, offset, bytes);
+    bdrv_dec_in_flight(bs);
+
+    return ret;
+}