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Subject: Re: [Xen-devel] [RFC PATCH 04/16] x86/xen: hypercall support for
 xenhost_t
To:     Andrew Cooper <andrew.cooper3@citrix.com>,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
Cc:     jgross@suse.com, sstabellini@kernel.org, konrad.wilk@oracle.com,
        pbonzini@redhat.com, boris.ostrovsky@oracle.com,
        joao.m.martins@oracle.com
References: <20190509172540.12398-1-ankur.a.arora@oracle.com>
 <20190509172540.12398-5-ankur.a.arora@oracle.com>
 <11f8b620-11ac-7075-019a-30d6bad7583c@citrix.com>
From:   Ankur Arora <ankur.a.arora@oracle.com>
Message-ID: <fbfc0a0c-3707-7f17-9f2a-6c9d2c7b05b1@oracle.com>
Date:   Fri, 14 Jun 2019 00:20:28 -0700
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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@oracle.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?

Ankur

> 
> ~Andrew
> 
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