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. > A guest may assign a > virtio device to nested guests, and would wish it confined by the > emulated iommu. Well, that would be nice, but it can't be done. It could be fixed, but it would be an incompatible change so it would need a new feature bit corresponding changes in the Linux driver to do the dma map/unmap if it accepts the "respect IOMMU" feature. > More generally, a guest sees a virtio device as just another pci device, > and has no way to tell that it bypasses the iommu. Well, except the fact that the driver knows its a virtio device, because it's a virtio driver. It's not like you can write a driver that uses PCI DMA without knowing the particulars of the device you're using. -- 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