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

[David Gibson]

On Wed, Oct 19, 2011 at 11:10:15AM +0200, Avi Kivity wrote:
> 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?

Yes, using a feature bit.

>  On x86 it will continue to work if we
> rewrite the spec in terms of pci dma, what about non-x86?

No, anything with a non-optional IOMMU will break horribly.  That's
why we need a feature bit.

> > >  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.

Well, sure, but my point is that I'm not volunteering for it.  Someone
who actually needs the feature can do the work.

-- 
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