On Tue, Sep 30, 2014 at 11:01:29AM -0700, Andy Lutomirski wrote: > On Tue, Sep 30, 2014 at 10:53 AM, Konrad Rzeszutek Wilk > <konrad.wilk@xxxxxxxxxx> wrote: > >> 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. > > > > <scratches his head> I don't follow. > > If you boot a Xen PV dom0 on QEMU master with -machine q35,iommu=on > and you add a virtio device, dom0 will end up with a PCI device that > does DMA to "machine" addresses. These addresses are not compatible > with the DMA API (which works with bus addresses), nor are they the > same as physical addresses. That is presumarily because the IOMMU assumes the virtio devices are real devices, not fake ones. > > So virtio in current kernels won't work for the same reason they never > work on Xen. But virtio-pci with my patches won't work either, > because they (or the Xen hypervisor) will try to program the IOMMU > with a non-identity mapping, causing everything to explode. > > Hacking up the virtio-pci driver to explicitly ask Xen for machine > addresses might work, but, at the very least, it will be a giant > security hole if anyone binds a virtio device to a domain other than > dom0 (which, again, is kind of the point of having an IOMMU). > > >> > >> 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.) > > > > Correction: > > - The Xen project is trying to kill some of the paravirts off. > > - KVM uses paravirts as well (and then added some) > > By "paravirt" I meant PV, where there's the weird physical/machine > address discrepancy that's visible to the guest. This is not to say > that Xen PVH wouldn't also be screwed running on QEMU master. > > --Andy _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
