Re: [Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO

[Avi Kivity]

Blue Swirl wrote:
> >  I don't understand.  It's not a device that needs bouncing, it's a
> > particular transfer.  This could be either due to the transfer targeting
> > mmio, or due to the transfer requiring a transformation.
> >     
>
> Should the bouncing be something more much complex, for example
> negotiated between the devices? Or maybe the devices set up a
> transforming and non-transforming channel (which the other end should
> be able to transform some more) and use them as needed?
>   

Yes.  We already have two cases:

- may do partial request: useful for block storage where requests can be 
huge
- need full request: networking, where you can't send or receive half a 
packet; on the other hand, packets are small (even with tso)

You're adding a third case, always bounce, when the data undergoes a 
transformation which can't happen in-place.

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