Re: [PATCH v10 3/3] xen/domain: use get_initial_domain_id() instead of open-coded 0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.06.2025 20:28, dmkhn@xxxxxxxxx wrote:
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -492,7 +492,7 @@ static int late_hwdom_init(struct domain *d)
>      struct domain *dom0;
>      int rv;
>  
> -    if ( d != hardware_domain || d->domain_id == 0 )
> +    if ( d != hardware_domain || d->domain_id == get_initial_domain_id() )
>          return 0;
>  
>      rv = xsm_init_hardware_domain(XSM_HOOK, d);
> @@ -501,7 +501,7 @@ static int late_hwdom_init(struct domain *d)
>  
>      printk("Initialising hardware domain %d\n", hardware_domid);
>  
> -    dom0 = rcu_lock_domain_by_id(0);
> +    dom0 = rcu_lock_domain_by_id(get_initial_domain_id());
>      ASSERT(dom0 != NULL);

For both changes above you're introducing a subtle (largely theoretical)
behavioral change: In shim mode, this assertion, if we somehow made it
here, would suddenly not trigger anymore. Similarly for the earlier
change the return path may wrongly be taken then.

> @@ -2479,7 +2479,7 @@ domid_t domid_alloc(domid_t domid)
>          if ( domid == DOMID_FIRST_RESERVED )
>              domid = find_next_zero_bit(domid_bitmap,
>                                         DOMID_FIRST_RESERVED,
> -                                       1);
> +                                       get_initial_domain_id() + 1);

This imo is the worst of the changes. get_initial_domain_id() can return
non-zero. In that case we still would (in principle) want to re-start
from 1 here.

Jan