Re: [Qemu-devel] [PATCH v6 16/18] qcow2: Switch store_bitmap_data() to byte-based iteration

[Eric Blake]

On 09/08/2017 08:27 AM, Kevin Wolf wrote:
> Am 30.08.2017 um 23:05 hat Eric Blake geschrieben:
>> Now that we have adjusted the majority of the calls this function
>> makes to be byte-based, it is easier to read the code if it makes
>> passes over the image using bytes rather than sectors.
>>
>> Signed-off-by: Eric Blake <address@hidden>
>> Reviewed-by: John Snow <address@hidden>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>

>>
>> -    while ((sector = bdrv_dirty_iter_next(dbi) >> BDRV_SECTOR_BITS) != -1) {
>> -        uint64_t cluster = sector / sbc;
>> +    while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
> 
> Don't you have to multiply both sides of the equation? This would be
> offset != -512, which points out that the previous patch to convert
> bdrv_dirty_iter_next() to byte-based gave it a really awkward interface.

I think what I really need to do is change '!= -1' to '< 0', as that's
much easier to reason about when scaling is present.

> 
>> +        uint64_t cluster = offset / limit;
>>          uint64_t end, write_size;
>>          int64_t off;
>>
>> -        sector = cluster * sbc;
>> -        end = MIN(bm_sectors, sector + sbc);
>> -        write_size = bdrv_dirty_bitmap_serialization_size(bitmap,
>> -            sector * BDRV_SECTOR_SIZE, (end - sector) * BDRV_SECTOR_SIZE);
>> +        offset = cluster * limit;
> 
> You just had cluster = offset / limit, so in other words, align down
> offset? If so, this is how it should be written.

Thanks for the close reviews; looks like I have enough things to do a
respin.

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

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