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.

Ok, so the next spin of these patches will have explicit LE and BE
versions of the accessors by popular demand.  I'm still using
cpu_physical_memory_rw() as the backend though, because I can't see a
case where a device could safely _require_ an emulated DMA access to
be atomic.

> Virtio is a very, very special case.  virtio requires coherent RAM access.

Right.  Virtio's access to memory is *not* emulated PCI DMA, it's
god-like hypervisor access to guest system memory.  It should
correctly bypass any IOMMU, and so should remain as
cpu_physical_memory_rw() or the atomic accessors, rather than being
converted to this new API.

-- 
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