On Mon, Sep 29, 2014 at 1:49 PM, Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote: > On Mon, 2014-09-29 at 11:55 -0700, Andy Lutomirski wrote: > >> Rusty and Michael, what's the status of this? > > The status is that I still think we need *a* way to actually inform the > guest whether the virtio implementation will or will not bypass the > IOMMU. I don't know Xen enough to figure out how to do that and we could > maybe just make it something qemu puts in the device-tree on powerpc > only. > > However I dislike making it global or per-bus, we could have a > combination of qemu and HW virtio on the same guest, so I really think > this needs to be a capability of the virtio device. Or a capability of the PCI slot, somehow. I don't understand PCI topology very well. > > I don't completely understand what games Xen is playing here, but from > what I can tell, it's pretty clear that today's qemu implementation > always bypasses any iommu and so should always be exported as such on > all platforms, at least all kvm and pure qemu ones. Except that I think that PPC is the only platform on which QEMU's code actually bypasses any IOMMU. Unless we've all missed something, there is no QEMU release that will put a virtio device behind an IOMMU on any platform other than PPC. If the eventual solution is to say that virtio 1.0 PCI devices always respect an IOMMU unless they set a magic flag saying "I'm not real hardware and I bypass the IOMMU", then I don't really object to that, except that it'll be a mess if the guest is running Xen. But even Xen would (I think) be okay if it actually worked by having a new DMA API operation that says "this device is magically identity mapped" and then just teaching Xen to implement that. But I'm not an OASIS member, so I can't really do this. I agree that this issue needs to be addressed somehow, but I don't think it needs to block these patches. --Andy > >> I think that (aside from the trivial DMI/DMA typo) the only real issue >> here is that the situation on PPC is ugly. We're failing to enable >> physical virtio hardware on PPC with these patches, but that never >> worked anyway. I don't think that there are any regressions other >> than ugliness. >> >> My preference would be to apply the patches as is (or with "DMA" >> spelled correctly), and then to: >> >> - Make sure that all virtio-mmio systems have working DMA ops so that >> virtio-mmio can the DMA API >> >> - Fix the DMA API on s390 (probably easy) and on PPC (not necessarily so easy) >> >> - Remove the non-DMA-API code, which would be a very small change on >> top of these patches. >> >> --Andy > > -- Andy Lutomirski AMA Capital Management, LLC _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
