Re: [PATCH v3] arinc653: don't assume Dom0 is the control domain

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.04.2026 14:57, Jürgen Groß wrote:
> On 01.04.26 14:29, Jan Beulich wrote:
>> Leaving aside highly disaggregated environments, the control domain is
>> what will invoke XEN_SYSCTL_SCHEDOP_putinfo. Its vCPU-s therefore need to
>> be able to run unconditionally, not those of the domain with ID 0 (which
>> may not exist at all).
>>
>> Fixes: 9f0c658baedc ("arinc: add cpu-pool support to scheduler")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> There being no "else" to the if(), what about other control domain vCPU-s?
> 
> I guess this is a stale leftover. Doesn't matter for committing anyway.
> 
>> ---
>> v3: Don't mistakenly include the idle domain.
>> v2: New.
>>
>> --- a/xen/common/sched/arinc653.c
>> +++ b/xen/common/sched/arinc653.c
>> @@ -411,10 +411,10 @@ a653sched_alloc_udata(const struct sched
>>       spin_lock_irqsave(&sched_priv->lock, flags);
>>   
>>       /*
>> -     * Add every one of dom0's units to the schedule, as long as there are
>> -     * slots available.
>> +     * Add every one of the control domain's units to the schedule, as long 
>> as
>> +     * there are slots available.
>>        */
>> -    if ( unit->domain->domain_id == 0 )
>> +    if ( is_control_domain(unit->domain) && !is_idle_domain(unit->domain) )
>>       {
>>           entry = sched_priv->num_schedule_entries;
>>   
> 
> Hmm, is it really the control domain only which wants to be scheduled 
> initially?
> I would think that at least the hardware domain and probably a Xenstore domain
> would want to be included, too.
> 
> In the end it might even be that other domains created via dom0less would want
> to be able to run initially. They could be part of a mandatory infrastructure.
> Why would they need to be created at boot if they are NOT important?

This part is easy to answer: Because in a dom0less setup you simply may have
no toolstack at all. (At which point there may also be nothing to set a
schedule, yes.)

> The question is whether the arinc653 scheduler is really meant for such 
> setups.
> OTOH just modifying the test to:
> 
>      if ( system_state < SYS_STATE_active &&
>           unit->domain->domain_id < DOMID_FIRST_RESERVED )
> 
> seems to be fine for catching all those cases.
> 
> With or without this modification:
> 
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

Thanks, yet I'll have to leave to the maintainers to decide which form it
should ultimately take. One remark: A restartable control domain wouldn't
pass that conditional. Granted that's looking far into the future.

Jan