Re: [Qemu-devel] [PATCH v6 12/24] block: Convert bdrv_get_block_status_above() to bytes

[Kevin Wolf <kwolf@redhat.com>]

Am 12.10.2017 um 05:47 hat Eric Blake geschrieben:
> We are gradually moving away from sector-based interfaces, towards
> byte-based.  In the common case, allocation is unlikely to ever use
> values that are not naturally sector-aligned, but it is possible
> that byte-based values will let us be more precise about allocation
> at the end of an unaligned file that can do byte-based access.
> 
> Changing the name of the function from bdrv_get_block_status_above()
> to bdrv_block_status_above() ensures that the compiler enforces that
> all callers are updated.  Likewise, since it a byte interface allows
> an offset mapping that might not be sector aligned, split the mapping
> out of the return value and into a pass-by-reference parameter.  For
> now, the io.c layer still assert()s that all uses are sector-aligned,
> but that can be relaxed when a later patch implements byte-based
> block status in the drivers.
> 
> For the most part this patch is just the addition of scaling at the
> callers followed by inverse scaling at bdrv_block_status(), plus
> updates for the new split return interface.  But some code,
> particularly bdrv_block_status(), gets a lot simpler because it no
> longer has to mess with sectors.  Likewise, mirror code no longer
> computes s->granularity >> BDRV_SECTOR_BITS, and can therefore drop
> an assertion about alignment because the loop no longer depends on
> alignment (never mind that we don't really have a driver that
> reports sub-sector alignments, so it's not really possible to test
> the effect of sub-sector mirroring).  Fix a neighboring assertion to
> use is_power_of_2 while there.
> 
> For ease of review, bdrv_get_block_status() was tackled separately.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

>  int bdrv_block_status(BlockDriverState *bs, int64_t offset, int64_t bytes,
>                        int64_t *pnum, int64_t *map, BlockDriverState **file)
>  {
> -    int64_t ret;
> -    int n;
> -
> -    assert(QEMU_IS_ALIGNED(offset | bytes, BDRV_SECTOR_SIZE));
> -    assert(pnum);
> -    /*
> -     * The contract allows us to return pnum smaller than bytes, even
> -     * if the next query would see the same status; we truncate the
> -     * request to avoid overflowing the driver's 32-bit interface.
> -     */
> -    bytes = MIN(bytes, BDRV_REQUEST_MAX_BYTES);

Is the limitation to BDRV_REQUEST_MAX_BYTES going away without being
replaced by a new one in bdrv_co_block_status()? What protects us now
from 32-bit truncation?

Kevin