Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA

[David Gibson]

On Mon, Oct 03, 2011 at 08:17:05AM -0500, Anthony Liguori wrote:
> On 10/02/2011 07:14 AM, Michael S. Tsirkin wrote:
> >On Sun, Oct 02, 2011 at 02:01:10PM +0200, Avi Kivity wrote:
> >>>Hmm, not entirely virtio specific, some devices use stX macros to do the
> >>>conversion.  E.g. stw_be_phys and stl_le_phys are used in several
> >>>places.
> >>
> >>These are fine - explicit endianness.
> >
> >Right. So changing these to e.g. stl_dma and assuming
> >LE is default seems like a step backwards.
> 
> We're generalizing too much.
> 
> In general, the device model doesn't need atomic access functions.
> That's because device model RAM access is not coherent with CPU RAM
> access.
> 
> Virtio is a very, very special case.  virtio requires coherent RAM
> access.

Right, but it should only need that for the actual rings in the virtio
core.  I was expecting that those would remain as direct physical
memory accesses - precisely because virtio is special - rather than
accesses through any kind of DMA interface.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson