On 10/18/2011 03:46 AM, David Gibson 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.
Wow, how did we manage to break virtio in so many different ways? Is there a way to unbreak it? On x86 it will continue to work if we rewrite the spec in terms of pci dma, what about non-x86? > > > 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. Needs to be done IMO. > > > 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. virtio-pci knows it's pci, there's no excuse. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
