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

[Juergen Gross <jgross@suse.com>]

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?

Juergen