On Tue, 18 Oct 2011 12:46:50 +1100, David Gibson <d...@au1.ibm.com> wrote: > On Sun, Oct 16, 2011 at 03:15:53PM +0200, Avi Kivity wrote: > > On 10/14/2011 04:14 AM, David Gibson wrote: > > > > 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. > > > > virtio should definitely not bypass an iommu. > > So, I just had a chat with Rusty about this. Perhaps it shouldn't, > but it does. The spec is in terms of guest physical addresses, not > bus/DMA addresses, and more to the point the Linux driver does *not* > do the necessary dma_map() and unmap operations to treat this as a PCI > DMA. So like it or not, god-like hypervisor access rather than > emulated PCI DMA is what it does.
Yep, it shouldn't but it does. Can't break it now without a feature bit, and there's no particular reason to add it until someone really wants it. Cheers, Rusty.
