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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 14, 2011 at 01:14:07PM +1100, David Gibson wrote:
> 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.

You don't?
PCI spec supports atomic operations. It also strongly recommends
not splitting accesses below dword boundary.

> > 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