Re: [Qemu-devel] [PATCH RFC] fixup! virtio: convert to use DMA api

[Andy Lutomirski <luto@amacapital.net>]

On Apr 20, 2016 6:14 AM, "Michael S. Tsirkin" <mst@redhat.com> 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@redhat.com> 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@redhat.com> 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@redhat.com> 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