Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool

[Anthony Liguori <anthony@codemonkey.ws>]

Andrea Arcangeli wrote:
> On Fri, Dec 12, 2008 at 11:25:55AM -0600, Anthony Liguori wrote:
>   
>> Hrm, that's more complex than I was expecting.  I was thinking the bdrv aio 
>> infrastructure would always take an iovec.  Any details about the 
>> underlying host's ability to handle the iovec would be insulated.
>>     
>
> You can't remove the restart memory-capped mechanism from the dma api,
> we've to handle dma to non-ram that potentially requires to copy the
> whole buffer so we're forced to have a safe linearization at the dma
> api layer. So it's not necessary to reinvent the same
> restart-partial-transfer logic in the aio layer too. Just set the
> define and teach the aio logic to use pread/pwrite if iovcnt == 1 and
> you're done ;).
>
> So what I'm suggesting is simpler than what you were expecting, not
> more complex. It would be more complex to replicate the restart-bounce
> logic in the aio layer too.
>
> Old drivers using bdrv_aio_read/write will keep working, new drivers
> using dma api can also use bdrv_aio_readv/writev and the linearization
> will happen inside the dma api if aio misses preadv/pwritev support.
>   

You assume that anything using bdrv_aio_readv/writev will be going 
through a DMA API.  This isn't a safe assumption.

Furthermore, you need some way to communicate the fact that you cannot 
handle iovcnt > 1 iovecs in a performant/safe way.

Regards,

Anthony Liguori