On Tue, Nov 10, 2015 at 09:37:54PM +1100, Benjamin Herrenschmidt wrote: > 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. Then we are back to virtio driver telling DMA core whether it wants a 1:1 mapping in the iommu? If that's acceptable to others, I don't think that's too bad. > 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. But not virtio-pci I think - that's broken for that usecase since we use weaker barriers than required for real IO, as these have measureable overhead. We could have a feature "is a real PCI device", that's completely reasonable. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
