On Thu, 12/07 14:30, Eric Blake wrote: > 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 <ebl...@redhat.com> > > --- > v6: drop now-useless rounding of mid-sector end-of-file hole [Kevin], > better documentation of 'want_zero' [Kevin]
Reviewed-by: Fam Zheng <f...@redhat.com>
