Re: [PATCH v5 2/3] virtio_pci: Use the DMA API for virtqueues when possible

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2014-09-24 at 14:41 -0700, Andy Lutomirski wrote:
> On Sat, Sep 20, 2014 at 10:05 PM, Benjamin Herrenschmidt
> <benh@kernel.crashing.org> wrote:
> > On Sun, 2014-09-21 at 15:03 +1000, Benjamin Herrenschmidt wrote:
> >
> >> The exception I mentioned is that I would really like the virtio device
> >> to expose via whatever transport we chose to use (though capability
> >> exchange sounds like a reasonable one) whether the "server"
> >> implementation is bypassing IOMMUs or not instead on relying on client
> >> side heuristics.
> >>
> >> IE. Basically, we are trying to "guess" with an ifdef CONFIG_PPC, what
> >> is essentially an attribute of the server-side, ie, whether is bypasses
> >> the iommu for the PCI bus it resides on.
> >
> >> I believe all the arguments about whether this should be a bus property
> >> or whether the x86 case can be worked around via ACPI tables etc... are
> >> all moot. Today, qemu implementation can put virtio devices on busses
> >> with an iommu and bypass it, so at the very least for backward
> >> compatibility, we should expose that appropriately from the "server"
> >> side.
> >
> > And of course, since we are talking about backward compatibility with
> > existing qemus here, the capability should be the opposite, ie "honor
> > iommu", with the assumption that without it, the implementation bypasses
> > it, which reflects what the current qemu implementation does on any
> > architecture, whether you configure the bus to have an iommu emulated on
> > it or not.
> 
> Can PPC do this using a new devicetree property?

The DT props for PCI devices are created by the FW inside the guest from
standard PCI probing, so it would have to get the info from qemu via
config or register space.

Cheers,
Ben.