Re: [PATCH v2 2/9] xen/arm: allocate static shared memory to the default owner dom_io

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.05.2022 09:24, Penny Zheng wrote:
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -777,8 +777,22 @@ void __init setup_system_domains(void)
>  
>      /*
>       * Initialise our DOMID_IO domain.
> -     * This domain owns I/O pages that are within the range of the page_info
> -     * array. Mappings occur at the priv of the caller.

This information is effectively lost; I cannot spot a suitable replacement
in the new text.

> +     * DOMID_IO is used for mapping memory and MMIO regions when no explicit
> +     * Domain need to be specified.
> +     *
> +     * For instance, DOMID_IO is the owner of memory pre-shared among
> +     * multiple domains at boot time, when no explicit owner is specified.
> +     *
> +     * Also, DOMID_IO is used to restrict page-table updates to mapping I/O
> +     * memory. Although no Foreign Domain need to be specified to map I/O
> +     * pages, DOMID_IO is useful to ensure that no mappings to the OS's own
> +     * heap are accidentally installed. (e.g., in Linux this could cause
> +     * havoc as reference counts aren't adjusted on the I/O-mapping code
> +     * path). This only makes sense as HYPERVISOR_mmu_update()'s and
> +     * HYPERVISOR_update_va_mapping_otherdomain()'s "foreigndom" argument.
> +     * For HYPERVISOR_mmu_update() context it can be specified by any
> +     * calling domain, otherwise it's only permitted if the caller is
> +     * privileged.

Much of this looks to have been copied from the public header. I don't
see why you did this; if anything a reference there would suffice.

>       * Quarantined PCI devices will be associated with this domain.
>       */

I would also like to ask that you add the new purpose of this domain
_below_ all pre-existing ones, such that this also somewhat expressed
how things have evolved.

Jan