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block: Clarify that @bytes is no limit on *pnum 

.bdrv_co_block_status() implementations are free to return a *pnum that
exceeds @bytes, because bdrv_co_block_status() in block/io.c will clamp
*pnum as necessary.

On the other hand, if drivers' implementations return values for *pnum
that are as large as possible, our recently introduced block-status
cache will become more effective.

So, make a note in block_int.h that @bytes is no upper limit for *pnum.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210812084148.14458-4-hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Hanna Reitz 2021-08-12 10:41:45 +02:00
parent 0bc329fbb0
commit 5a1cfd2150
1 changed files with 9 additions and 0 deletions
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9
include/block/block_int.h
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@ -348,6 +348,15 @@ struct BlockDriver {
* clamped to bdrv_getlength() and aligned to request_alignment,
* as well as non-NULL pnum, map, and file; in turn, the driver
* must return an error or set pnum to an aligned non-zero value.
*
* Note that @bytes is just a hint on how big of a region the
* caller wants to inspect. It is not a limit on *pnum.
* Implementations are free to return larger values of *pnum if
* doing so does not incur a performance penalty.
*
* block/io.c's bdrv_co_block_status() will utilize an unclamped
* *pnum value for the block-status cache on protocol nodes, prior
* to clamping *pnum for return to its caller.
*/
int coroutine_fn (*bdrv_co_block_status)(BlockDriverState *bs,
bool want_zero, int64_t offset, int64_t bytes, int64_t *pnum,