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

[Kevin Wolf <kwolf@redhat.com>]

Am 11.08.2011 17:05, schrieb Paolo Bonzini:
> On 08/11/2011 04:37 PM, Kevin Wolf wrote:
>>>  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).
>>
>> Isn't the DMAAIOCB already passed as opaque to the callback?
> 
> It is passed to the dma_bdrv_cb, but not to the caller-provided 
> callback.  If the operation completes before dma_bdrv_{read,write} 
> returns, the AIOCB is not stored anywhere and the asynchronous callback 
> does not have access to it.  Usually it does not have anything to do 
> with it, but in this case it could get the residual.
> 
> Another possibility is always completing DMA in a bottom half.  This 
> ensures that the callback can access the AIOCB, but it exposes an 
> implementation detail to the caller, so I don't like it.

At least in the block layer, AIO callbacks may never be called before
the submission function has returned. I think this makes the DMA helpers
provide the same behaviour.

But I'm not sure if the definition of the AIOCB struct isn't private to
the block layer.

Kevin