On Fri, Apr 18, 2025 at 04:52:46PM -0500, Eric Blake wrote: > This patch is purely mechanical, changing bool want_zero into an > unsigned int for bitwise-or of flags. As of this patch, all > implementations are unchanged (the old want_zero==true is now > mode==BDRV_WANT_PRECISE which is a superset of BDRV_WANT_ZERO); but > the callers in io.c that used to pass want_zero==false are now > prepared for future driver changes that can now distinguish bewteen > BDRV_WANT_ZERO vs. BDRV_WANT_ALLOCATED. The next patch will actually > change the file-posix driver along those lines, now that we have > more-specific hints. > > As for the background why this patch is useful: right now, the > file-posix driver recognizes that if allocation is being queried, the > entire image can be reported as allocated (there is no backing file to > refer to) - but this throws away information on whether the entire > image reads as zero (trivially true if lseek(SEEK_HOLE) at offset 0 > returns -ENXIO, a bit more complicated to prove if the raw file was > created with 'qemu-img create' since we intentionally allocate a small > chunk of all-zero data to help with alignment probing). Later patches > will add a generic algorithm for seeing if an entire file reads as > zeroes. > > Signed-off-by: Eric Blake <ebl...@redhat.com> > --- > > In response to Stefan's ask for what it would look like as a bitmask > of flags instead of an enum. Only the first two patches of the series > change.
> +++ b/block/io.c > @@ -2364,10 +2364,8 @@ int bdrv_flush_all(void) > * 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, 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. > + * 'mode' serves as a hint as to which results are favored; see enum > + * BlockStatusMode for details of the supported modes. Stale comment applicable to v2 but not v2.5; will fix for v3. -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org
