On 24.06.21 12:25, Vladimir Sementsov-Ogievskiy wrote:
24.06.2021 13:16, Max Reitz wrote:
On 24.06.21 11:15, Vladimir Sementsov-Ogievskiy wrote:
23.06.2021 18:01, Max Reitz wrote:
.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 <ebl...@redhat.com>
Signed-off-by: Max Reitz <mre...@redhat.com>
---
  include/block/block_int.h | 5 +++++
  1 file changed, 5 insertions(+)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index fcb599dd1c..f85b96ed99 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -347,6 +347,11 @@ 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.

Worth mention that the cache will benefit of it?

Oh, right, absolutely.  Like so:

"block/io.c's bdrv_co_block_status() will clamp *pnum before returning it to its caller, but it itself can still make use of the unclamped *pnum value.  Specifically, the block-status cache for protocol nodes will benefit from storing as large a region as possible."


Sounds good. Do you mean this as an addition or substitution? If the latter, I'd keep "if doing so does not incur a performance penalty

I meant it as an addition, just a new paragraph.

Max


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