Re: [Qemu-devel] [PATCH] block/io: optimize bdrv_co_pwritev for small requests

[Peter Lieven <pl@kamp.de>]

Am 24.05.2016 um 15:59 schrieb Paolo Bonzini:
>
> On 24/05/2016 15:39, Peter Lieven wrote:
>>           bytes += offset & (align - 1);
>>           offset = offset & ~(align - 1);
> Because the low bits have been masked away from offset and added to bytes,
>
>> +
>> +        /* if head and tail fall into the same alignment
>> +         * we can omit the second read as it would read
>> +         * the same block again */
>> +        if ((offset + bytes) & (align - 1) &&
> ... the first part is just "bytes & (align - 1)"...
>
>> +            offset / align == (offset + bytes) / align) {
> ... and the second part is just "bytes < align" (you can distribute
> division over addition because offset / align has no reminder, and
> simplify to "0 == bytes / align").
>
> Putting it together, it becomes "bytes > 0 && bytes < align", or even
> "bytes < align".

Oh, thanks, and the if block also too complicated. If I am right it should
collapse to:

         if (bytes < align) {
             qemu_iovec_add(&local_qiov, head_buf + bytes,
                            align - bytes);
             bytes = align;
         }

Right?

Thanks,
Peter