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

[Eric Blake <eblake@redhat.com>]

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On 10/20/2017 10:03 AM, Kevin Wolf wrote:
> 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.
>>

>>  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?

As of this patch, we have this 64-bit-clean call chain:
bdrv_block_status()
  bdrv_block_status_above()
    bdrv_common_block_status_above()
      bdrv_block_status_above_co_entry()
        bdrv_co_block_status_above()
          bdrv_co_block_status()

which in turn has:
    /*
     * 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);

added earlier in 8/24.

At the end of series 3, the truncation prevention is still in place when
calling into the drivers, per the changes to bdrv_co_block_status() in
21/24.

Later, in series 4, all drivers are converted to 64-bit interfaces,
where the drivers are either made 64-bit clean, or do their own
truncation to 32 bits.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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