Re: [Xen-devel] [RFC PATCH 04/16] x86/xen: hypercall support for xenhost_t

[Ankur Arora]

On 2019-06-12 2:15 p.m., Andrew Cooper wrote:
> On 09/05/2019 18:25, Ankur Arora wrote:
> > Allow for different hypercall implementations for different xenhost types.
> > Nested xenhost, which has two underlying xenhosts, can use both
> > simultaneously.
> >
> > The hypercall macros (HYPERVISOR_*) implicitly use the default xenhost.x
> > A new macro (hypervisor_*) takes xenhost_t * as a parameter and does the
> > right thing.
> >
> > TODO:
> >    - Multicalls for now assume the default xenhost
> >    - xen_hypercall_* symbols are only generated for the default xenhost.
> >
> > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
>
> Again, what is the hypervisor nesting and/or guest layout here?
Two hypervisors, L0 and L1, and the guest is a child of the L1
hypervisor but could have PV devices attached to both L0 and L1
hypervisors.

> I can't think of any case where a single piece of software can
> legitimately have two hypercall pages, because if it has one working
> one, it is by definition a guest, and therefore not privileged enough to
> use the outer one.
Depending on which hypercall page is used, the hypercall would
(eventually) land in the corresponding hypervisor.

Juergen elsewhere pointed out proxying hypercalls is a better approach,
so I'm not really considering this any more but, given this layout, and
assuming that the hypercall pages could be encoded differently would it
still not work?

Ankur

> ~Andrew
>
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