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

[Juergen Gross <jgross@xxxxxxxx>]

On 14.06.19 09:20, Ankur Arora wrote:
> 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?
Hypercalls might work, but it is a bad idea and a violation of layering
to let a L1 guest issue hypercalls to L0 hypervisor, as those hypercalls
could influence other L1 guests and even the L1 hypervisor.

Hmm, thinking more about it, I even doubt those hypercalls could work in
all cases: when issued from a L1 PV guest the hypercalls would seem to
be issued from user mode for the L0 hypervisor, and this is not allowed.


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel