Re: [Qemu-devel] [PATCH 1/5] Add target memory mapping API

[Ian Jackson]

Avi Kivity writes ("Re: [Qemu-devel] [PATCH 1/5] Add target memory mapping 
API"):
> The devices we're talking about here don't have a DMA count register. 

Which devices ?  All devices ever that want to do zero-copy DMA into
the guest ?

> They are passed scatter-gather lists, and I don't think they make 
> guarantees about the order in which they're accessed.

Many devices will be able to make such promises.

> This DMA will be into RAM, not mmio.

As previously discussed, we might be using bounce buffers even for
RAM, depending on the execution model.  You said earlier:

  Try it out. I'm sure it will work just fine (if incredibly slowly, 
  unless you provide multiple bounce buffers).

but here is an example from Jamie of a situation where it won't work
right.

> We don't have a reliable amount to pass.

A device which _really_ doesn't have a reliable amount to pass, and
which is entitled to scribble all over the RAM it was to DMA to even
if it does only a partial transfer, can simply pass the total transfer
length.  That would be no different to your proposal.

Ian.