Re: [PATCH] x86/hvm: Allow the guest to permit the use of userspace hypercalls

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Wed, 13 Jan 2016, Juergen Gross wrote:
> On 13/01/16 11:41, Stefano Stabellini wrote:
> > On Wed, 13 Jan 2016, Juergen Gross wrote:
> >> On 12/01/16 18:23, Stefano Stabellini wrote:
> >>> On Tue, 12 Jan 2016, Juergen Gross wrote:
> >>>> On 12/01/16 18:05, Stefano Stabellini wrote:
> >>>>> On Tue, 12 Jan 2016, Jan Beulich wrote:
> >>>>>>>>> On 12.01.16 at 13:07, <stefano.stabellini@eu.citrix.com> wrote:
> >>>>>>> On Mon, 11 Jan 2016, David Vrabel wrote:
> >>>>>>>> On 11/01/16 17:17, Andrew Cooper wrote:
> >>>>>>>>> So from one point of view, sufficient justification for this change is
> >>>>>>>>> "because the Linux way isn't the only valid way to do this".
> >>>>>>>>
> >>>>>>>> "Because we can" isn't a good justification for adding something new.
> >>>>>>>> Particularly something that is trivially easy to (accidentally) misuse
> >>>>>>>> and open a big security hole between userspace and kernel.
> >>>>>>>>
> >>>>>>>> The vague idea for a userspace netfront that's floating around
> >>>>>>>> internally is also not a good reason for pushing this feature at this time.
> >>>>>>>
> >>>>>>> I agree with David, but I might have another good use case for this.
> >>>>>>>
> >>>>>>> Consider the following scenario: we have a Xen HVM guest, with Xen
> >>>>>>> installed inside of it (nested virtualization). I'll refer to Xen
> >>>>>>> running on the host as L0 Xen and Xen running inside the VM as L1 Xen.
> >>>>>>> Similarly we have two dom0 running, the one with access to the physical
> >>>>>>> hardware, L0 Dom0, and the one running inside the VM, L1 Dom0.
> >>>>>>>
> >>>>>>> Let's suppose that we want to lay the groundwork for L1 Dom0 to use PV
> >>>>>>> frontend drivers, netfront and blkfront to speed up execution. In order
> >>>>>>> to do that, the first thing it needs to do is making an hypercall to L0
> >>>>>>> Xen. That's because netfront and blkfront needs to communicate with
> >>>>>>> netback and blkback in L0 Dom0: event channels and grant tables are the
> >>>>>>> ones provided by L0 Xen.
> >>>>>>
> >>>>>> That's again a layering violation (bypassing the L1 hypervisor).
> >>>>>
> >>>>> True, but in this scenario it might be necessary for performance
> >>>>> reasons: otherwise every hypercall would need to bounce off L1 Xen,
> >>>>> possibly cancelling the benefits of running netfront and blkfront in the
> >>>>> first place. I don't have numbers though.
> >>>>
> >>>> How is this supposed to work? How can dom0 make hypercalls to L1 _or_ L0
> >>>> hypervisor? How can it select the hypervisor it is talking to?
> >>>
> >>> >From L0 Xen point of view, the guest is just a normal PV on HVM guest,
> >>> it doesn't matter what's inside, so L1 Dom0 is going to make hypercalls
> >>> to L0 Xen like any other PV on HVM guests: mapping the hypercall page by
> >>> writing to the right MSR, retrieved via cpuid, then calling into the
> >>
> >> But how to specify that cpuid/MSR should target the L0 hypervisor
> >> instead of L1?
> > 
> > Keeping in mind that L1 Dom0 is a PV guest from L1 Xen point of view,
> > but a PV on HVM guest from L0 Xen point of view, it is true that the
> > cpuid could be an issue because the cpuid would be generated by L0 Xen,
> > but then would get filtered by L1 Xen. However the MSR should be OK,
> > assuming that L1 Xen allows access to it: from inside the VM it would
> > look like a regular machine MSR, it couldn't get confused with anything
> > causing hypercalls to L1 Xen.
> 
> L1 Xen wouldn't allow access to it. Otherwise it couldn't ever setup
> a hypercall page for one of it's guests.

If they are PV guests, the hypercall page is not mapped via MSRs.


> >> And even if this would be working, just by mapping
> >> the correct page the instructions doing the transition to the
> >> hypervisor would still result in entering the L1 hypervisor, as
> >> those instructions must be handled by L1 first in order to make
> >> nested virtualization work.
> > 
> > This is wrong. The hypercall page populated by L0 Xen would contain
> > vmcall instructions. When L1 Dom0 calls into the hypercall page, it
> > would end up making a vmcall, which brings it directly to L0 Xen,
> > skipping L1 Xen.
> 
> Sure. And L0 Xen will see that this guest is subject to nested
> virtualization and is reflecting the vmcall to L1 Xen (see e.g.
> xen/arch/x86/hvm/svm/nestedsvm.c, nestedsvm_check_intercepts() ).
> How else would L1 Xen ever get a vmcall of one of it's guests?

Only if nested_hvm is enabled, I believe. Sorry for not being clearer
earlier: I am talking about nested virtualization without nested
vmx/svm, which is the default.

But you have a good point there: if nested_hvm is enabled, there should
still be a way for L1 Dom0 to issue an hypercall to L0 Xen, otherwise
how could L1 Dom0 ever setup netfront and blkfront?