So ... I've finally tried to sort that out for powerpc and I can't find a way to make that work that isn't a complete pile of stinking shit.
I'm very tempted to go back to my original idea: virtio itself should indicate it's "bypassing ability" via the virtio config space or some other bit (like the ProgIf of the PCI header etc...). I don't see how I can make it work otherwise. The problem with the statement "it's a platform matter" is that: - It's not entirely correct. It's actually a feature of a specific virtio implementation (qemu's) that it bypasses all the platform IOMMU facilities. - The platforms (on powerpc there's at least 3 or 4 that have qemu emulation and support some form of PCI) don't have an existing way to convey the information that a device bypasses the IOMMU (if any). - Even if we add such a mechanism (new property in the device-tree), we end up with a big turd: Because we need to be compatible with older qemus, we essentially need a quirk that will make all virtio devices assume that property is present. That will of course break whenever we try to use another implementation of virtio on powerpc which doesn't bypass the iommu. We don't have a way to differenciate a virtio device that does the bypass from one that doesn't and the backward compatibility requirement forces us to treat all existing virtio devices as doing bypass. 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 ? Cheers, Ben. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html