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
