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

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

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

Cheers,
Ben.