Re: [Qemu-block] [Qemu-devel] [RFC PATCH 28/56] block: Widen dirty bitmap granularity to uint64_t for safety

[Eric Blake]

On 08/07/2017 08:55 PM, John Snow wrote:
> 
> 
> On 08/07/2017 10:45 AM, Markus Armbruster wrote:
>> Block dirty bitmaps represent granularity in bytes as uint32_t.  It
>> must be a power of two and a multiple of BDRV_SECTOR_SIZE.
>>
>> The trouble with uint32_t is computations like this one in
>> mirror_do_read():
>>
>>     uint64_t max_bytes;
>>
>>     max_bytes = s->granularity * s->max_iov;
>>
>> The operands of * are uint32_t and int, so the product is computed in
>> uint32_t (assuming 32 bit int), then zero-extended to uint64_t.
>>
>> Since granularity is generally combined with 64 bit file offsets, it's
>> best to make it 64 bits, too.  Less opportunity to screw up.

And definitely conflicts with my work on byte-based block status.


>>  
>> -uint32_t bdrv_dirty_bitmap_meta_granularity(BdrvDirtyBitmap *bitmap)
>> -{
>> -    return BDRV_SECTOR_SIZE << hbitmap_granularity(bitmap->meta);
>> -}
> 
> Why? Unused? Not cool enough to mention?

Already deleted as unused in my byte-based series.

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

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