Re: [PATCH v8 3/3] xen/domain: introduce CONFIG_MAX_DOMID

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.05.2025 02:00, dmkhn@xxxxxxxxx wrote:
> --- a/xen/arch/arm/tee/ffa.c
> +++ b/xen/arch/arm/tee/ffa.c
> @@ -331,10 +331,9 @@ static int ffa_domain_init(struct domain *d)
>       * reserved for the hypervisor and we only support secure endpoints using
>       * FF-A IDs with BIT 15 set to 1 so make sure those are not used by Xen.
>       */
> -    BUILD_BUG_ON(DOMID_FIRST_RESERVED >= UINT16_MAX);

Why's this being moved to common code? It certainly may have a purpose here
(which I'm simply unaware of); I don't see what purpose it has in common
code.

> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -576,4 +576,11 @@ config BUDDY_ALLOCATOR_SIZE
>         Amount of memory reserved for the buddy allocator to serve Xen heap,
>         working alongside the colored one.
>  
> +config MAX_DOMID
> +     int "Maximum number of user domains"
> +     range 1 32752
> +     default 32752
> +     help
> +       Specifies the maximum number of domains a user can create.

My prior comment remains: The description and help needs to be accurate, in
order to not cause any confusion. In a true dom0less environment I'm not
sure the "user" can create any domains (post boot, that is). And when there
is Dom0 (or late hwdom), the number specified already isn't the number of
domains one can create (again, post boot, which is how I understand "user
domains"). If someone picked 1 as the value here, it's unclear to me how
late hwdom or dom0less would work in the first place.

> --- a/xen/include/public/domctl.h
> +++ b/xen/include/public/domctl.h
> @@ -36,7 +36,7 @@
>  
>  /*
>   * NB. xen_domctl.domain is an IN/OUT parameter for this operation.
> - * If it is specified as an invalid value (0 or >= DOMID_FIRST_RESERVED),
> + * If it is specified as an invalid value (0 or >= CONFIG_MAX_DOMID),

In the public interface I question the relevance of any implementation
details of the hypervisor.

Jan