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

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxxxxxxx> 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.

>> 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?


Juergen

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