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

[Anthony Liguori]

Avi Kivity wrote:
> Anthony Liguori wrote:
> > Paul Brook wrote:
> > > It looks like what you're actually doing is pushing the bounce 
> > > buffer allocation into the individual packet consumers.
> > >
> > > Maybe a solution to this is a 'do IO on IOVEC' actor, with an 
> > > additional flag that says whether it is acceptable to split the 
> > > allocation. That way both block and packet interfaces use the same 
> > > API, and avoids proliferation of manual bounce buffers in packet 
> > > devices.
> > >   
> >
> > I think there may be utility in having packet devices provide the 
> > bounce buffers, in which case, you could probably unique both into a 
> > single function with a flag.  But why not just have two separate 
> > functions?
> >
> > Those two functions can live in exec.c too.  The nice thing about 
> > using map() is that it's easily overriden and chained.  So what I'm 
> > proposing.
> >
> > cpu_physical_memory_map()
> > cpu_physical_memory_unmap()
>
> This should be the baseline API with the rest using it.

Yup.

> > do_streaming_IO(map, unmap, ioworker, opaque);
>
> Why pass map and unmap?

Because we'll eventually have:

pci_device_memory_map()
pci_device_memory_unmap()

In the simplest case, pci_device_memory_map() just calls 
cpu_physical_memory_map().  But it may do other things.

> grant based devices needn't go through this at all, since you never 
> mix grants and physical addresses, and since grants never need bouncing.

So the grant map/unmap function doesn't need to deal with calling 
cpu_physical_memory_map/unmap.  You could still use the above API or 
not.  It's hard to say.

> > do_packet_IO(map, unmap, buffer, size, ioworker, opaque);
>
> If you pass the buffer then the device needs to allocate large amounts 
> of bounce memory.

If do_packet_IO took a buffer, instead of calling alloc_buffer(size) 
when map fails (you run out of bounce memory), you simply use buffer.  
Otherwise, alloc_buffer() must be able to allocate enough memory to 
satisfy any request.

Since each packet device knows it's maximum size up front, it makes 
sense for the device to allocate it.  You could also not care and just 
trust that callers do the right thing.

Regards,

Anthony Liguori