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? -- 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
