Re: [PATCH v4] xen/domain: introduce DOMID_ANY

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.02.2026 10:25, Juergen Gross wrote:
> On 04.02.26 10:15, Jan Beulich wrote:
>> On 04.02.2026 10:00, Roger Pau Monné wrote:
>>> On Wed, Feb 04, 2026 at 08:56:10AM +0100, Jan Beulich wrote:
>>>> On 04.02.2026 08:49, Roger Pau Monné wrote:
>>>>> Also, I would remove the tools guards, I think once a DOMID_ constant
>>>>> is allocated it becomes part of the public ABI, and it cannot be
>>>>> withdrawn.  See for example DOMID_IDLE: it's only used internally in
>>>>> the hypervisor AFAICT, yet the define is fully visible in the
>>>>> headers.
>>>>
>>>> It was me to ask for it to be guarded like this. DOMID_IDLE (and perhaps
>>>> others) not being guarded (at least for IDLE: by just __XEN__) imo was a
>>>> mistake. That mistake may in fact be correctable, if we could prove that
>>>> the ID cannot usefully be passed into anywhere.
>>>
>>> Even if it's not passed into anything, does it make sense to guard
>>> them?  The reserved domid values are already consumed, ie: cannot be
>>> reused in any way.  It just seem to me like more ifdefery churn for no
>>> specific benefit.
>>
>> Well. From an abstract perspective, purely hypothetical at this point, I
>> could see a potential need to re-number them, e.g. to simplify checking
>> against groups of these special IDs.
>>
>> Yes, excess #ifdef-ary is an issue. Excess exposure of things also is,
>> though. Finding the right balance between both can be interesting.
> 
> I have a patch in work which would want DOMID_ANY not be guarded. I think
> especially DOMID_ANY could be useful for other cases, too.

Mind me asking where, outside of the toolstack, you intend to use it? Note
that unlike IDLE and perhaps a few others, ...

> Look e.g. at XEN_ARGO_DOMID_ANY. Even if I don't think we can switch it
> to DOMID_ANY, it shows that the concept as such is not restricted to Xen
> internal use cases.

... I don't mean ANY to be constrained to just the hypervisor. That more
strict guarding came up only because of Roger mentioning IDLE.

Jan