Re: [Qemu-devel] [PATCH 05/10] dma-helpers: add dma_buf_read and dma_buf_write

[Paolo Bonzini]

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