Re: [PATCH v4 0/6] virtio core DMA API conversion

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

On Mon, 2015-11-09 at 16:46 -0800, Andy Lutomirski wrote:
> The problem here is that in some of the problematic cases the virtio
> driver may not even be loaded.  If someone runs an L1 guest with an
> IOMMU-bypassing virtio device and assigns it to L2 using vfio, then
> *boom* L1 crashes.  (Same if, say, DPDK gets used, I think.)
> 
> >
> > The only way out of this while keeping the "platform" stuff would be to
> > also bump some kind of version in the virtio config (or PCI header). I
> > have no other way to differenciate between "this is an old qemu that
> > doesn't do the 'bypass property' yet" from "this is a virtio device
> > that doesn't bypass".
> >
> > Any better idea ?
> 
> I'd suggest that, in the absence of the new DT binding, we assume that
> any PCI device with the virtio vendor ID is passthrough on powerpc.  I
> can do this in the virtio driver, but if it's in the platform code
> then vfio gets it right too (i.e. fails to load).

The problem is there isn't *a* virtio vendor ID. It's the RedHat vendor
ID which will be used by more than just virtio, so we need to
specifically list the devices.

Additionally, that still means that once we have a virtio device that
actually uses the iommu, powerpc will not work since the "workaround"
above will kick in.

The "in absence of the new DT binding" doesn't make that much sense.

Those platforms use device-trees defined since the dawn of ages by
actual open firmware implementations, they either have no iommu
representation in there (Macs, the platform code hooks it all up) or
have various properties related to the iommu but no concept of "bypass"
in there.

We can *add* a new property under some circumstances that indicates a
bypass on a per-device basis, however that doesn't completely solve it:

  - As I said above, what does the absence of that property mean ? An
old qemu that does bypass on all virtio or a new qemu trying to tell
you that the virtio device actually does use the iommu (or some other
environment that isn't qemu) ?

  - On things like macs, the device-tree is generated by openbios, it
would have to have some added logic to try to figure that out, which
means it needs to know *via different means* that some or all virtio
devices bypass the iommu.

I thus go back to my original statement, it's a LOT easier to handle if
the device itself is self describing, indicating whether it is set to
bypass a host iommu or not. For L1->L2, well, that wouldn't be the
first time qemu/VFIO plays tricks with the passed through device
configuration space...

Note that the above can be solved via some kind of compromise: The
device self describes the ability to honor the iommu, along with the
property (or ACPI table entry) that indicates whether or not it does.

IE. We could use the revision or ProgIf field of the config space for
example. Or something in virtio config. If it's an "old" device, we
know it always bypass. If it's a new device, we know it only bypasses
if the corresponding property is in. I still would have to sort out the
openbios case for mac among others but it's at least a workable
direction.

BTW. Don't you have a similar problem on x86 that today qemu claims
that everything honors the iommu in ACPI ?

Unless somebody can come up with a better idea...

Cheers,
Ben.