Re: [Qemu-devel] [PATCH v6 14/18] qcow2: Switch qcow2_measure() to byte-based iteration

[Eric Blake <eblake@redhat.com>]

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On 09/08/2017 08:15 AM, Kevin Wolf wrote:
> Am 30.08.2017 um 23:05 hat Eric Blake geschrieben:
>> This is new code, but it is easier to read if it makes passes over
>> the image using bytes rather than sectors (and will get easier in
>> the future when bdrv_get_block_status is converted to byte-based).
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>> Reviewed-by: John Snow <jsnow@redhat.com>
>>

>> -                int nb_sectors = MIN(ssize / BDRV_SECTOR_SIZE - sector_num,
>> -                                     BDRV_REQUEST_MAX_SECTORS);
>> +            for (offset = 0; offset < ssize;
>> +                 offset += pnum * BDRV_SECTOR_SIZE) {
>> +                int nb_sectors = MIN(ssize - offset,
>> +                                     INT_MAX) / BDRV_SECTOR_SIZE;
> 
> Shouldn't this be BDRV_REQUEST_MAX_BYTES? (Which is close to INT_MAX,
> but rounded down to sector alignment.)

The division rounds down to sector alignment after the MIN(), for the
same result in nb_sectors either way.  But you are correct that an
absolutely literal translation of the pre-patch version would feed
BDRV_REQUEST_MAX_BYTES instead of INT_MAX into the MIN().

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


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