Re: [PATCH] arinc653: clear entire .dom_handle[] for Dom0 slots

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 3/24/26 11:54, Jan Beulich wrote:
> When that code still lived in a653sched_init(), it was redundant with the
> earlier memset() / xzalloc(). Once moved, the full structure field needs
> setting, as dom_handle_cmp() uses memcmp().

The whole a653sched_priv_t *sched_priv is still allocated in a653sched_init()
with xzalloc(), so it's still redundant post-move. With that said, the code is
only setting the first element (of an already-zeroed array), which is suspicious
and misleading. What we really should be doing here is copy unit->domain->handle
to sched_priv->schedule[entry].dom_handle.

> 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 Dom0 vCPU-s?

The condition is checking minor frame entries (i.e. available slots in the
schedule). Once those are exhausted, Dom0 units beyond
ARINC653_MAX_DOMAINS_PER_SCHEDULE would never be scheduled. Currently the
scheduler only supports scheduling a single unit at a time, so this way of
adding units to the schedule would result in Dom0 vCPUs being scheduled
sequentially, which is incredibly inefficient and unlikely to be what anyone
would actually want, but I don't think there are other possibilities given the
current lack of multicore support.

There was an effort some time ago to introduce multicore scheduling, see [1].
I'd be happy to review if somebody wants to pick that up again.

[1] 
https://lore.kernel.org/xen-devel/20200916181854.75563-1-jeff.kubascik@xxxxxxxxxxxxxxx/T/#t

> And why
> is it that domain ID 0 is special here, rather than the hardware and/or
> control domain(s)? (Likely the latter as that's what would invoke
> XEN_SYSCTL_SCHEDOP_putinfo, and hence needs to be able to run without
> that having been issued first.)

This likely should be updated to is_control_domain(unit->domain).

> 
> --- a/xen/common/sched/arinc653.c
> +++ b/xen/common/sched/arinc653.c
> @@ -420,7 +420,8 @@ a653sched_alloc_udata(const struct sched
>  
>          if ( entry < ARINC653_MAX_DOMAINS_PER_SCHEDULE )
>          {
> -            sched_priv->schedule[entry].dom_handle[0] = '\0';
> +            memset(sched_priv->schedule[entry].dom_handle, '\0',
> +                   sizeof(sched_priv->schedule[entry].dom_handle));
>              sched_priv->schedule[entry].unit_id = unit->unit_id;
>              sched_priv->schedule[entry].runtime = DEFAULT_TIMESLICE;
>              sched_priv->schedule[entry].unit = unit;
>