On Nov 10, 2015 2:38 AM, "Benjamin Herrenschmidt"
[off-list ref] wrote:

On Mon, 2015-11-09 at 21:35 -0800, Andy Lutomirski wrote:

quoted

We could do it the other way around: on powerpc, if a PCI device is in
that range and doesn't have the "bypass" property at all, then it's
assumed to bypass the IOMMU.  This means that everything that
currently works continues working.  If someone builds a physical
virtio device or uses another system in PCIe target mode speaking
virtio, then it won't work until they upgrade their firmware to set
bypass=0.  Meanwhile everyone using hypothetical new QEMU also gets
bypass=0 and no ambiguity.

vfio will presumably notice the bypass and correctly refuse to map any
current virtio devices.

Would that work?

That would be extremely strange from a platform perspective. Any device
in that vendor/device range would bypass the iommu unless some new
property "actually-works-like-a-real-pci-device" happens to exist in
the device-tree, which we would then need to define somewhere and
handle accross at least 3 different platforms who get their device-tree
from widly different places.

Also if tomorrow I create a PCI device that implements virtio-net and
put it in a machine running IBM proprietary firmware (or Apple's or
Sun's), it won't have that property...

This is not hypothetical. People are using virtio to do point-to-point
communication between machines via PCIe today.

Does that work on powerpc on existing kernels?

Anyway, here's another crazy idea: make the quirk assume that the
IOMMU is bypasses if and only if the weak barriers bit is set on
systems that are missing the new DT binding.

--Andy

Cheers,
Ben.