Re: [Qemu-devel] [PATCH 2/6] PCI DMA API

[Anthony Liguori]

Paul Brook wrote:
> On Sunday 30 March 2008, Anthony Liguori wrote:
>   
> The entity processing the data shouldn't need to know or care how the 
> translation is done. PhysIOVector should describe everything it need to know.
>   

Okay, I'll update.

> > What could work is if the DMA API functions mapped PhysIOVector =>
> > PhysIOVector and then the network and block subsystems could operate on
> > a PhysIOVector.  I have patches that implement vector IO for net and
> > block but didn't want to include them in this series to keep things simple.
> >     
>
> IMHO this is the only sane way to implement zero-copy.
>
>   
> > > > This enables zero-copy IO to be preformed without introducing
> > > > assumptions of phys_ram_base.  This API is at the PCI device level to
> > > > enable support of per-device IOMMU remapping.
> > > >         
> > > By my reading it *requires* bridges be zero-copy.  For big-endian targets
> > > we need to ability to byteswap accesses.
> > >       
> > You mean via ld/st_phys?  
> >     
>
> By whatever means the bridge deems necessary. The whole point of the DMA API 
> is that you're transferring a block of data. The API allows intermediate 
> busses to transform that data (and address) without the block handler needing 
> to know or care.
>
> With your current scheme a byteswapping bus has to allocate a single large 
> buffer for the whole vector, even if the device then ends up copying unto a 
> local buffer in small chunks.
>   

Oh, I see now.  The DMA API should have not just a mechanism to do bulk 
transfers but also provide an interface to do load/store's that could 
potentially be byte-swapped.  I didn't realize buses did that.

Regards,

Anthony Liguori

> Paul
>