Re: virtio DMA API?

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 27, 2014 at 08:40:51PM +0930, Rusty Russell wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
> > Currently, a lot of the virtio code assumes that bus (i.e. hypervisor)
> > addresses are the same as physical address.  This is false on Xen, so
> > virtio is completely broken.  I wouldn't be surprised if it also
> > becomes a problem the first time that someone sticks a physical
> > "virtio" device on a 32-bit bus on an ARM SOC with more than 4G RAM.
> >
> > Would you accept patches to convert virtio_ring and virtio_pci to use
> > the DMA APIs?  I think that the only real catch will be that
> > virtio_ring's approach to freeing indirect blocks is currently
> > incompatible with the DMA API -- it assumes that knowing the bus
> > address is enough to call kfree, and I don't think that the DMA API
> > provides a reverse mapping like that.
> 
> Hi Andy,
> 
>         This has long been a source of contention.  virtio assumes that
> the hypervisor can decode guest-physical addresses.
> 
>         PowerPC, in particular, doesn't want to pay the cost of IOMMU
> manipulations, and all arguments presented so far for using an IOMMU for
> a virtio device are weak.  And changing to use DMA APIs would break them
> anyway.
> 
>         Of course, it's Just A Matter of Code, so it's possible to
> create a Xen-specific variant which uses the DMA APIs.  I'm not sure
> what that would look like in the virtio standard, however.
> 
> Cheers,
> Rusty.

For x86 as of QEMU 2.0 there's no iommu.
So a reasonable thing to do for that platform
might be to always use iommu *if it's there*.
My understanding is this isn't the case for powerpc?

-- 
MST