Re: [PATCH v2 04/17] xen/riscv: construct the P2M pages pool for guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.06.2025 15:05, Oleksii Kurochko wrote:
> @@ -113,3 +117,58 @@ int p2m_init(struct domain *d)
>  
>      return 0;
>  }
> +
> +/*
> + * Set the pool of pages to the required number of pages.
> + * Returns 0 for success, non-zero for failure.
> + * Call with d->arch.paging.lock held.
> + */
> +int p2m_set_allocation(struct domain *d, unsigned long pages, bool 
> *preempted)
> +{
> +    struct page_info *pg;
> +
> +    ASSERT(spin_is_locked(&d->arch.paging.lock));
> +
> +    for ( ; ; )
> +    {
> +        if ( d->arch.paging.p2m_total_pages < pages )
> +        {
> +            /* Need to allocate more memory from domheap */
> +            pg = alloc_domheap_page(d, MEMF_no_owner);
> +            if ( pg == NULL )
> +            {
> +                printk(XENLOG_ERR "Failed to allocate P2M pages.\n");
> +                return -ENOMEM;
> +            }
> +            ACCESS_ONCE(d->arch.paging.p2m_total_pages)++;
> +            page_list_add_tail(pg, &d->arch.paging.p2m_freelist);
> +        }
> +        else if ( d->arch.paging.p2m_total_pages > pages )
> +        {
> +            /* Need to return memory to domheap */
> +            pg = page_list_remove_head(&d->arch.paging.p2m_freelist);
> +            if( pg )
> +            {
> +                ACCESS_ONCE(d->arch.paging.p2m_total_pages)--;
> +                free_domheap_page(pg);
> +            }
> +            else
> +            {
> +                printk(XENLOG_ERR
> +                       "Failed to free P2M pages, P2M freelist is empty.\n");
> +                return -ENOMEM;
> +            }
> +        }
> +        else
> +            break;
> +
> +        /* Check to see if we need to yield and try again */
> +        if ( preempted && general_preempt_check() )
> +        {
> +            *preempted = true;
> +            return -ERESTART;
> +        }
> +    }
> +
> +    return 0;
> +}

Btw, with the order-2 requirement for the root page table, you may want to
consider an alternative approach: Here you could allocate some order-2
pages (possibly up to as many as a domain might need, which right now
would be exactly one), put them on a separate list, and consume the root
table(s) from there. If you run out of pages on the order-0 list, you
could shatter a page from the order-2 one (as long as that's still non-
empty). The difficulty would be with freeing, where a previously shattered
order-2 page would be nice to re-combine once all of its constituents are
free again. The main benefit would be avoiding the back and forth in patch
6.

Jan