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

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Date:   Fri, 14 Jun 2019 09:35:23 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Ankur Arora <ankur.a.arora@...cle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:     sstabellini@...nel.org, konrad.wilk@...cle.com,
        pbonzini@...hat.com, boris.ostrovsky@...cle.com,
        joao.m.martins@...cle.com
Subject: Re: [Xen-devel] [RFC PATCH 04/16] x86/xen: hypercall support for
 xenhost_t

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@...cle.com>
>>
>> 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