On Tue, Sep 30, 2014 at 08:48:45AM -0700, Andy Lutomirski wrote: > On Tue, Sep 30, 2014 at 8:38 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > I thought hard about this, I think we are better off waiting till the > > next release: there's a chance QEMU will have IOMMU support for KVM x86 > > then, and this will make it easier to judge which way does the wind > > blow. > > > > It seems that we lose nothing substantial keeping the status quo a bit longer, > > but if we make an incompatible change in guests now we might > > create nasty compatibility headaches going forward. > > > > I would argue for the opposite approach. Having a QEMU release that > supports an IOMMU on x86 and exposes a commonly used PCI device that > bypasses that IOMMU without any explicit notification to the guest > (and specification!) that this is happening is IMO insane. Once that > happens, we'll have to address the nasty case on both x86 and PPC. > This will suck. > > If we accept the guest change and make sure that there is never a QEMU > release that has a visible IOMMU cheat on any arch other than PPC, > then at least the damage will be contained. Wrt QEMU this sounds reasonable. Wrt guest, deferring guest changes a bit, until we have a better idea about how the host side behaves sounds better to me than saying "this is how guests will behave, let the host adapt to that". > x86 will be worse than PPC, too: the special case needed to support > QEMU 2.2 with IOMMU and virtio enabled with a Xen guest will be fairly > large and disgusting and will only exist to support something that IMO > should never have existed in the first place. > > PPC at least avoids *that* problem by virtue of not having Xen > paravirt. (And please don't add Xen paravirt to PPC -- x86 is trying > to kill it off, but this is a 5-10 year project.) > > [..., reordered] > > >> > >> 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. > > > > I think that is true but it seems that this will be true for x86 for > > QEMU 2.2 unless we make some changes there. > > Which we might not have the time for since 2.2 is feature frozen > > from tomorrow. > > Maybe we should disable the IOMMU in 2.2, this is worth considering. > > > > Please do. Or at least disable it just if there are virtio devices. > Also, try booting this 2.2 QEMU candidate with nested virtualization > on. Then bind vfio to a virtio-pci device and watch the guest get > corrupted. QEMU will blame Linux for incorrectly programming the > hardware, and Linux will blame QEMU for its blatant violation of the > ACPI spec. Given that this is presumably most of the point of adding > IOMMU support, it seems like a terrible idea to let code like that > into the wild. > > If this happens, Linux may also end up needing a quirk to prevent vfio > from binding to QEMU 2.2's virtio-pci devices. > > --Andy This specific item wouldn't worry me too much. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
