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 <[email protected]> >>
>> {
>> 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 <[email protected]>
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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