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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 7/1/25 3:04 PM, Jan Beulich wrote:

> 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.
>     
It is an option.

But I'm still not 100% sure it's necessary to allocate the root page table
from the freelist. We could simply allocate the root page table from the
domheap (as is done for hardware domains) and reserve the freelist for other
pages.
The freelist is specific to Dom0less guest domains and is primarily used to
limit the amount of memory available for the guest—essentially for static
configurations where you want a clear and fixed limit on p2m allocations.

~ Oleksii