Re: [Xen-devel] [PATCH mini-os enhancements for vtpm 2/8] add posix io to blkfront

[Matthew Fioravante <matthew.fioravante@xxxxxxxxxx>]

On 09/17/2012 06:46 PM, Samuel Thibault wrote:
> Matthew Fioravante, le Mon 17 Sep 2012 17:55:43 -0400, a écrit :
>> +   /* Read mode checks */
>> +   else
>> +   {
>> +      /*If the requested read is bigger than the disk, just
>> +       * read as much as we can until the end */
>> +      if(offset + count > disksize) {
>> +         count = offset >= disksize ? 0 : disksize - offset;
>> +      }
>> +   }
> Perhaps return 0 here already instead of just setting count to 0?
I agree
>
>> +
>> +   /* Setup aiocb block object */
>> +   aiocb.aio_dev = dev;
>> +   aiocb.aio_nbytes = blocksize;
>> +   aiocb.aio_offset = blknum * blocksize;
>> +   aiocb.aio_cb = NULL;
>> +   aiocb.data = NULL;
>> +
>> +   /* If our buffer is unaligned or its aligned but we will need to rw
>> a partial block
>> +    * then a copy will have to be done */
>> +   if(!alignedbuf || blkoff != 0 || count % blocksize != 0) {
>> +      copybuf = _xmalloc(blocksize, dev->info.sector_size);
>> +   }
>> +
>> +   rc = count;
>> +   while(count > 0) {
>> +      /* determine how many bytes to read/write from/to the current
>> block buffer */
>> +      bytes = count > (blocksize - blkoff) ? blocksize - blkoff : count;
> Mmm. Optimizing the non-aligned case would make the code
> tricky, but shouldn't optimizing the aligned case at least
> a little bit not too hard? i.e. set bytes to max(count,
> BLKIF_MAX_SEGMENTS_PER_REQUEST*PAGE_SIZE); in the optimized case? That'd
> get much better 44KiB transfers instead of one sector at a time.
>
> Ideally we should even push a series of aio requests, but that's a lot
> less easy since you need to know how many requests you can afford.
See latest v2 patch
> Apart from that,
>
> Acked-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>

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