Re: [PATCH v3 1/2] xen: Add capabilities to get_domain_state

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.07.2025 08:55, Jürgen Groß wrote:
> On 23.07.25 08:43, Jan Beulich wrote:
>> On 23.07.2025 08:34, Jürgen Groß wrote:
>>> On 23.07.25 08:28, Jan Beulich wrote:
>>>> On 22.07.2025 02:19, Jason Andryuk wrote:
>>>>> --- a/xen/common/domain.c
>>>>> +++ b/xen/common/domain.c
>>>>> @@ -195,6 +195,14 @@ static void set_domain_state_info(struct 
>>>>> xen_domctl_get_domain_state *info,
>>>>>            info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_DYING;
>>>>>        if ( d->is_dying == DOMDYING_dead )
>>>>>            info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_DEAD;
>>>>> +
>>>>> +    info->caps = 0;
>>>>> +    if ( is_control_domain(d) )
>>>>> +        info->caps |= XEN_DOMCTL_GETDOMSTATE_CAP_CONTROL;
>>>>> +    if ( is_hardware_domain(d) )
>>>>> +        info->caps |= XEN_DOMCTL_GETDOMSTATE_CAP_HARDWARE;
>>>>> +    if ( is_xenstore_domain(d) )
>>>>> +        info->caps |= XEN_DOMCTL_GETDOMSTATE_CAP_XENSTORE;
>>>>>        info->unique_id = d->unique_id;
>>>>>    }
>>>>
>>>> This being a stable sub-op, don't we need a way to indicate to the caller
>>>> _that_ this field has valid data now? When non-zero it's easy to tell, but
>>>> getting back zero is ambiguous.
>>>
>>> The hypercall sub-op was introduced in this development cycle only, so
>>> there is no released Xen hypervisor without the capability setting.
>>>
>>> In case this patch doesn't make it into 4.21, the case you are mentioning
>>> must be handled, of course.
>>
>> Hmm, yes, this way it's on the edge. As a stable sub-op, someone could have
>> backported the change, though. IOW (and in general) I wonder whether for
>> stable sub-ops we shouldn't be pretty strict about functional extensions,
>> not tying their addition to releases at all.
> 
> Should we really care about downstreams backporting not yet released
> functionality?
> 
> Using your reasoning this would mean we'd need to issue XSAs for not yet
> released hypervisor issues of stable interfaces, too. I don't think we
> want to do that.

To me, the latter doesn't necessarily follow from the former. But anyway, I'm
not going to insist, but I wanted the situation to at least be considered. In
particular also forward-looking, when we may gain more stable sub-ops. At some
point it may turn out necessary to backport such even into upstream branches.

Jan