On 08/11/2011 03:29 PM, Stefan Hajnoczi wrote:
>
> What do you think about passing the residual bytes for short transfers?
> Should I look into updating BlockDriverCompletionFunc, or is the approach
> of patch 2 okay? If I have an excuse to learn more about Coccinelle, that
> can be fun.:)
The bdrv_aio_readv() and bdrv_aio_writev() functions don't have the
concept of residual bytes. They only work on fully completed I/O
operations. If there is an error they pass -errno.
But if a transfer was split due to failure of cpu_physical_memory_map,
and only the second part fails, you can have a short transfer and you
need to pass residual bytes back. The only way out of this is to make a
bounce buffer as big as all the unmappable parts of the S/G list, which
is undesirable of course. So the residual bytes are a general DMA
concept, not specific to SCSI.
Therefore I don't think BlockDriverCompletionFunc is the right type
to add residual bytes to.
Right, I would rather update BlockDriverCompletionFunc to pass the AIOCB
as a third parameter, and store the residual bytes in the DMAAIOCB (with
a getter that the completion function can use).
Paolo
