On Apr 20, 2016 6:14 AM, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > On Tue, Apr 19, 2016 at 02:07:01PM -0700, Andy Lutomirski wrote: > > On Tue, Apr 19, 2016 at 1:54 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > On Tue, Apr 19, 2016 at 01:27:29PM -0700, Andy Lutomirski wrote: > > >> On Tue, Apr 19, 2016 at 1:16 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > >> > On Tue, Apr 19, 2016 at 11:01:38AM -0700, Andy Lutomirski wrote: > > >> >> On Tue, Apr 19, 2016 at 10:49 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > >> >> > On Tue, Apr 19, 2016 at 12:26:44PM -0400, David Woodhouse wrote: > > >> >> >> On Tue, 2016-04-19 at 19:20 +0300, Michael S. Tsirkin wrote: > > >> >> >> > > > >> >> >> > > I thought that PLATFORM served that purpose. Woudn't the host > > >> >> >> > > advertise PLATFORM support and, if the guest doesn't ack it, the host > > >> >> >> > > device would skip translation? Or is that problematic for vfio? > > >> >> >> > > > >> >> >> > Exactly that's problematic for security. > > >> >> >> > You can't allow guest driver to decide whether device skips security. > > >> >> >> > > >> >> >> Right. Because fundamentally, this *isn't* a property of the endpoint > > >> >> >> device, and doesn't live in virtio itself. > > >> >> >> > > >> >> >> It's a property of the platform IOMMU, and lives there. > > >> >> > > > >> >> > It's a property of the hypervisor virtio implementation, and lives there. > > >> >> > > >> >> It is now, but QEMU could, in principle, change the way it thinks > > >> >> about it so that virtio devices would use the QEMU DMA API but ask > > >> >> QEMU to pass everything through 1:1. This would be entirely invisible > > >> >> to guests but would make it be a property of the IOMMU implementation. > > >> >> At that point, maybe QEMU could find a (platform dependent) way to > > >> >> tell the guest what's going on. > > >> >> > > >> >> FWIW, as far as I can tell, PPC and SPARC really could, in principle, > > >> >> set up 1:1 mappings in the guest so that the virtio devices would work > > >> >> regardless of whether QEMU is ignoring the IOMMU or not -- I think the > > >> >> only obstacle is that the PPC and SPARC 1:1 mappings are currectly set > > >> >> up with an offset. I don't know too much about those platforms, but > > >> >> presumably the layout could be changed so that 1:1 really was 1:1. > > >> >> > > >> >> --Andy > > >> > > > >> > Sure. Do you see any reason why the decision to do this can't be > > >> > keyed off the virtio feature bit? > > >> > > >> I can think of three types of virtio host: > > >> > > >> a) virtio always bypasses the IOMMU. > > >> > > >> b) virtio never bypasses the IOMMU (unless DMAR tables or similar say > > >> it does) -- i.e. virtio works like any other device. > > >> > > >> c) virtio may bypass the IOMMU depending on what the guest asks it to do. > > > > > > d) some virtio devices bypass the IOMMU and some don't, > > > e.g. it's harder to support IOMMU with vhost. > > > > > > > > >> If this is keyed off a virtio feature bit and anyone tries to > > >> implement (c), the vfio is going to have a problem. And, if it's > > >> keyed off a virtio feature bit, then (a) won't work on Xen or similar > > >> setups unless the Xen hypervisor adds a giant and probably unreliable > > >> kludge to support it. Meanwhile, 4.6-rc works fine under Xen on a > > >> default x86 QEMU configuration, and I'd really like to keep it that > > >> way. > > >> > > >> What could plausibly work using a virtio feature bit is for a device > > >> to say "hey, I'm a new device and I support the platform-defined IOMMU > > >> mechanism". This bit would be *set* on default IOMMU-less QEMU > > >> configurations and on physical virtio PCI cards. > > > > > > And clear on xen. > > > > How? QEMU has no idea that the guest is running Xen. > > I was under impression xen_enabled() is true in QEMU. > Am I wrong? I'd be rather surprised, given that QEMU would have to inspect the guest kernel to figure it out. I'm talking about Xen under QEMU. For example, if you feed QEMU a guest disk image that contains Fedora with the xen packages installed, you can boot it and get a grub menu. If you ask grub to boot Xen, you get Xen. If you ask grub to boot Linux directly, you don't get Xen. I assume xen_enabled is for QEMU under Xen, i.e. QEMU, running under Xen, supplying emulated devices to a Xen domU guest. Since QEMU is seeing the guest address space directly, this should be much the same as QEMU !xen_enabled -- if you boot plain Linux, everything works, but if you do Xen -> QEMU -> HVM guest running Xen PV -> Linux, then virtio drivers in the Xen PV Linux guest need to translate addresses. --Andy > > -- > MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
