block: Add .bdrv_co_block_status() callback

We are gradually moving away from sector-based interfaces, towards
byte-based. Now that the block layer exposes byte-based allocation,
it's time to tackle the drivers.  Add a new callback that operates
on as small as byte boundaries. Subsequent patches will then update
individual drivers, then finally remove .bdrv_co_get_block_status().

The new code also passes through the 'want_zero' hint, which will
allow subsequent patches to further optimize callers that only care
about how much of the image is allocated (want_zero is false),
rather than full details about runs of zeroes and which offsets the
allocation actually maps to (want_zero is true).  As part of this
effort, fix another part of the documentation: the claim in commit
4c41cb4 that BDRV_BLOCK_ALLOCATED is short for 'DATA || ZERO' is a
lie at the block layer (see commit e88ae2264), even though it is
how the bit is computed from the driver layer.  After all, there
are intentionally cases where we return ZERO but not ALLOCATED at
the block layer, when we know that a read sees zero because the
backing file is too short.  Note that the driver interface is thus
slightly different than the public interface with regards to which
bits will be set, and what guarantees are provided on input.

We also add an assertion that any driver using the new callback will
make progress (the only time pnum will be 0 is if the block layer
already handled an out-of-bounds request, or if there is an error);
the old driver interface did not provide this guarantee, which
could lead to some inf-loops in drastic corner-case failures.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 19 insertions, 9 deletions

diff --git a/block/io.c b/block/io.c
index 89d0745e95..b00c7e2e2c 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1899,10 +1899,10 @@ int64_t coroutine_fn bdrv_co_get_block_status_from_backing(BlockDriverState *bs,
  * Drivers not implementing the functionality are assumed to not support
  * backing files, hence all their sectors are reported as allocated.
  *
- * If 'want_zero' is true, the caller is querying for mapping purposes,
- * and the result should include BDRV_BLOCK_OFFSET_VALID and
- * BDRV_BLOCK_ZERO where possible; otherwise, the result may omit those
- * bits particularly if it allows for a larger value in 'pnum'.
+ * If 'want_zero' is true, the caller is querying for mapping
+ * purposes, with a focus on valid BDRV_BLOCK_OFFSET_VALID, _DATA, and
+ * _ZERO where possible; otherwise, the result favors larger 'pnum',
+ * with a focus on accurate BDRV_BLOCK_ALLOCATED.
  *
  * If 'offset' is beyond the end of the disk image the return value is
  * BDRV_BLOCK_EOF and 'pnum' is set to 0.
@@ -1959,7 +1959,7 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
 
     /* Must be non-NULL or bdrv_getlength() would have failed */
     assert(bs->drv);
-    if (!bs->drv->bdrv_co_get_block_status) {
+    if (!bs->drv->bdrv_co_get_block_status && !bs->drv->bdrv_co_block_status) {
         *pnum = bytes;
         ret = BDRV_BLOCK_DATA | BDRV_BLOCK_ALLOCATED;
         if (offset + bytes == total_size) {
@@ -1976,13 +1976,14 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
     bdrv_inc_in_flight(bs);
 
     /* Round out to request_alignment boundaries */
-    /* TODO: until we have a byte-based driver callback, we also have to
-     * round out to sectors, even if that is bigger than request_alignment */
-    align = MAX(bs->bl.request_alignment, BDRV_SECTOR_SIZE);
+    align = bs->bl.request_alignment;
+    if (bs->drv->bdrv_co_get_block_status && align < BDRV_SECTOR_SIZE) {
+        align = BDRV_SECTOR_SIZE;
+    }
     aligned_offset = QEMU_ALIGN_DOWN(offset, align);
     aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset;
 
-    {
+    if (bs->drv->bdrv_co_get_block_status) {
         int count; /* sectors */
         int64_t longret;
 
@@ -2007,6 +2008,15 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
         }
         ret = longret & ~BDRV_BLOCK_OFFSET_MASK;
         *pnum = count * BDRV_SECTOR_SIZE;
+    } else {
+        ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset,
+                                            aligned_bytes, pnum, &local_map,
+                                            &local_file);
+        if (ret < 0) {
+            *pnum = 0;
+            goto out;
+        }
+        assert(*pnum); /* The block driver must make progress */
     }
 
     /*