Re: [PATCH v2 13/17] xen/riscv: Implement p2m_entry_from_mfn() and support PBMT configuration

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 7/23/25 11:46 AM, Jan Beulich wrote:

> > I assume that I have in this case to take some pages for an intermediate page
> > table from freelist P2M pool, set an owner domain to NULL (pg->inuse.domain = NULL).
> >
> > Then in this case it isn't clear why pg->list can't be re-used to link several pages
> > for intermediate page table purposes + metadata? Is it because pg->list can be not
> > empty? In this case it isn't clear if I could use a page, which has threaded pages.
> Actually looks like I was mis-remembering. Pages removed from freelist indeed
> aren't put on any other list, so the linking fields are available for use. I
> guess I had x86 shadow code in mind, where the linking fields are further used.
>     
Perhaps, I misunderstood you about "linking fields", but it seems like I can't reuse
struct page_info->list as it is used by page_list_add() which is called by p2m_alloc_page()
to allocate page(s) for an intermediate page table:
   static inline void
   page_list_add(struct page_info *page, struct page_list_head *head)
   {
        list_add(&page->list, head);
   }

    struct page_info * paging_alloc_page(struct domain *d)
    {
        struct page_info *pg;

        spin_lock(&d->arch.paging.lock);
        pg = page_list_remove_head(&d->arch.paging.freelist);
        spin_unlock(&d->arch.paging.lock);

        INIT_LIST_HEAD(&pg->list);

        return pg;
    }

    static struct page_info *p2m_alloc_page(struct domain *d)
    {
        struct page_info *pg = paging_alloc_page(d);

        if ( pg )
            page_list_add(pg, &p2m_get_hostp2m(d)->pages);

        return pg;
    }

So I have to reuse another field from struct page_info. It seems like it won't be an
issue if to add a new struct page_list_entry metadata_list to 'union v':
    union {
        /* Page is in use */
        struct {
            /* Owner of this page (NULL if page is anonymous). */
            struct domain *domain;
        } inuse;

        /* Page is on a free list. */
        struct {
            /* Order-size of the free chunk this page is the head of. */
            unsigned int order;
        } free;
+
+       struct page_list_entry metadata_list;       
    } v;

Am I missing something?

~ Oleksii