Re: [Qemu-devel] [PATCH v6 09/20] parallels: Switch to .bdrv_co_block_status()

[Eric Blake]

On 12/09/2017 06:31 AM, Vladimir Sementsov-Ogievskiy wrote:
> 07.12.2017 23:30, Eric Blake wrote:
>> We are gradually moving away from sector-based interfaces, towards
>> byte-based.  Update the parallels driver accordingly.  Note that
>> the internal function block_status() is still sector-based, because
>> it is still in use by other sector-based functions; but that's okay
>> because request_alignment is 512 as a result of those functions.
>> For now, no optimizations are added based on the mapping hint.
>>
>> Signed-off-by: Eric Blake <address@hidden>
>>

>>   {
>>       BDRVParallelsState *s = bs->opaque;
>> -    int64_t offset;
>> +    int count;
>>
>> +    assert(QEMU_IS_ALIGNED(offset | bytes, BDRV_SECTOR_SIZE));
>>       qemu_co_mutex_lock(&s->lock);
>> -    offset = block_status(s, sector_num, nb_sectors, pnum);
>> +    offset = block_status(s, offset >> BDRV_SECTOR_BITS,
>> +                          bytes >> BDRV_SECTOR_BITS, &count);
>>       qemu_co_mutex_unlock(&s->lock);
>>
>> +    *pnum = count * BDRV_SECTOR_SIZE;
>>       if (offset < 0) {
>>           return 0;
>>       }
>>
>> +    *map = offset;
> 
> oh, didn't notice previous time :(, should be
> 
> *map = offset * BDRV_SECTOR_SIZE
>

Oh, right.

> (also, if you already use ">> BDRV_SECTOR_BITS" in this function,
> would not it be more consistent to use "<< BDRV_SECTOR_BITS"
> for sector-to-byte conversion?)
> 
> with that fixed (with "<<" or "*", up to you),

'>> BDRV_SECTOR_BITS' always works.  You could also write '/
BDRV_SECTOR_SIZE' (the compiler should figure out that you are dividing
by a power of 2, and use the shift instruction under the hood), but I
find that a bit harder to reason about.

On the other hand, '<< BDRV_SECTOR_BITS' only produces the same size
output as the input; if the left side is 32 bits, it risks overflowing.
But '* BDRV_SECTOR_SIZE' always produces a 64-bit value.  So I've
learned (from past mistakes in other byte-conversion series) that the
multiply form is less likely to introduce unintended truncation bugs.

> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> 

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

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