Re: [PATCH v2 2/5] Mini-OS: get own domid

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.11.2023 10:12, Julien Grall wrote:
> 
> 
> On 13/11/2023 07:37, Jan Beulich wrote:
>> On 10.11.2023 18:38, Julien Grall wrote:
>>> Hi Jan,
>>>
>>> On 10/11/2023 12:44, Jan Beulich wrote:
>>>> On 10.11.2023 13:23, Roger Pau Monné wrote:
>>>>> On Fri, Nov 10, 2023 at 12:34:32PM +0100, Juergen Gross wrote:
>>>>>> Get the own domid via creation of a temporary event channel. There is
>>>>>> no "official" way to read the own domid in PV guests, so use the event
>>>>>> channel interface to get it:
>>>>>>
>>>>>> - allocate an unbound event channel specifying DOMID_SELF for the
>>>>>>     other end
>>>>>>
>>>>>> - read the event channel status which will contain the own domid in
>>>>>>     unbound.dom
>>>>>>
>>>>>> - close the event channel
>>>>>
>>>>> Should we look into introducing a way to expose the domid, so that in
>>>>> the future we might not need to resort to this workarounds to get the
>>>>> domid?
>>>>>
>>>>> Maybe in the PV-specific cpuid leaf?  It's a shame we didn't put it in
>>>>> a non-HVM specific leaf when it was made available to HVM for pvshim
>>>>> reasons.
>>>>
>>>> Couldn't we retroactively generalize the type-agnostic parts of that
>>>> leaf?
>>>
>>> This would only work for x86. I think we want to have a generic
>>> hypercalls so it can be used by all arch.
>>
>> Hmm, yes, perhaps. Otoh it would seem desirable to me if arch-es also
>> provided some extension to an arch-natural way of feature detection
>> (which CPUID is on x86), without the need to invoke any hypercalls.
> 
> For Arm, I can't really think of anything other than hvc/smc which are 
> used for calls to the hypervisor/monitor (so basically hypercalls).
> 
> Please suggest if you have a better idea.

I don't know enough Arm to properly suggest something. Arm64 has various
id_* system registers, so I would be wondering whether having a properly
virtual one reserved in system register space couldn't do the trick.
(Clearly just one may not be sufficient long term, so there might be a
scalability issue with such an approach. Unlike x86'es CPUID Arm's MRS,
aiui, doesn't take inputs other than the coordinates of the system
register to read.)

Jan