Re: [PATCH v4 10/18] xen/riscv: implement p2m_set_range()

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 9/20/25 1:36 AM, Jan Beulich wrote:

> On 17.09.2025 23:55, Oleksii Kurochko wrote:
> > --- a/xen/arch/riscv/p2m.c
> > +++ b/xen/arch/riscv/p2m.c
> > @@ -16,6 +16,7 @@
> >  #include <asm/riscv_encoding.h>
> >  
> >  unsigned long __ro_after_init gstage_mode;
> > +unsigned int __ro_after_init gstage_root_level;
> >  
> >  void __init gstage_mode_detect(void)
> >  {
> > @@ -53,6 +54,7 @@ void __init gstage_mode_detect(void)
> >          if ( MASK_EXTR(csr_read(CSR_HGATP), HGATP_MODE_MASK) == mode )
> >          {
> >              gstage_mode = mode;
> > +            gstage_root_level = modes[mode_idx].paging_levels - 1;
> >              break;
> >          }
> >      }
> > @@ -210,6 +212,9 @@ int p2m_init(struct domain *d)
> >      rwlock_init(&p2m->lock);
> >      INIT_PAGE_LIST_HEAD(&p2m->pages);
> >  
> > +    p2m->max_mapped_gfn = _gfn(0);
> > +    p2m->lowest_mapped_gfn = _gfn(ULONG_MAX);
> > +
> >      /*
> >       * Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
> >       * is not ready for RISC-V support.
> > @@ -251,13 +256,287 @@ int p2m_set_allocation(struct domain *d, unsigned long pages, bool *preempted)
> >      return rc;
> >  }
> >  
> > +/*
> > + * Find and map the root page table. The caller is responsible for
> > + * unmapping the table.
> >       
> With the root table being 4 pages, "the root table" is slightly misleading
> here: Yu never map the entire table.
>     
I will update the comment then to:
/*
 * Map one of the four root pages of the P2M root page table.
 *
 * The P2M root page table is larger than normal (16KB instead of 4KB),
 * so it is allocated as four consecutive 4KB pages. This function selects
 * the appropriate 4KB page based on the given GFN and returns a mapping
 * to it. 
 *
 * The caller is responsible for unmapping the page after use.
 *
 * Returns NULL if the calculated offset into the root table is invalid.
 */

> > + * The function will return NULL if the offset into the root table is
> > + * invalid.
> > + */
> > +static pte_t *p2m_get_root_pointer(struct p2m_domain *p2m, gfn_t gfn)
> > +{
> > +    unsigned long root_table_indx;
> > +
> > +    root_table_indx = gfn_x(gfn) >> P2M_LEVEL_ORDER(P2M_ROOT_LEVEL);
> > +    if ( root_table_indx >= P2M_ROOT_PAGES )
> > +        return NULL;
> > +
> > +    /*
> > +     * The P2M root page table is extended by 2 bits, making its size 16KB
> > +     * (instead of 4KB for non-root page tables). Therefore, p2m->root is
> > +     * allocated as four consecutive 4KB pages (since alloc_domheap_pages()
> > +     * only allocates 4KB pages).
> > +     *
> > +     * To determine which of these four 4KB pages the root_table_indx falls
> > +     * into, we divide root_table_indx by
> > +     * P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1).
> > +     */
> > +    root_table_indx /= P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1);
> >       
> The subtraction of 1 here feels odd: You're after the root table's
> number of entries, i.e. I'd expect you to pass just P2M_ROOT_LEVEL.
> And the way P2M_PAGETABLE_ENTRIES() works also suggests so.
>     
The purpose of this line is to select the page within the root table, which
consists of 4 consecutive pages. However, P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL)
returns 2048, so root_table_idx will always be 0 after devision, which is not
what we want.

As an alternative, P2M_PAGETABLE_ENTRIES(0) could be used, since it always
returns 512. Dividing root_table_idx by 512 then yields the index of the page
within the root table, which is made up of 4 consecutive pages.

Does it make sense now?

The problem may occur with DECLARE_OFFSET(), which can produce an incorrect
index within the root page table. Since the index is in the range [0, 2047],
it becomes an issue if the value is greater than 511, because DECLARE_OFFSET()
does not account for the larger range of the root index.

I am not sure whether it is better to make DECLARE_OFFSET() generic enough
for both P2M and Xen page tables, or to provide a separate P2M_DECLARE_OFFSET()
and use it only in P2M-related code.
Also, it could be an option to move DECLARE_OFFSET() from asm/page.h header
to riscv/pt.c and define another one DECLARE_OFFSETS in riscv/p2m.c.

Do you have a preference?

> > +/*
> > + * Insert an entry in the p2m. This should be called with a mapping
> > + * equal to a page/superpage.
> > + */
> >       
> I don't follow this comment: There isn't any mapping being passed in, is there?
>     
I think this comment should be dropped, it was about that requested mapping
should be equal to a page/superpage(4k, 2m, 1g), the correct order is always
guaranteed by p2m_mapping_order().

> > +static int p2m_set_entry(struct p2m_domain *p2m,
> > +                           gfn_t gfn,
> > +                           unsigned long page_order,
> > +                           mfn_t mfn,
> > +                           p2m_type_t t)
> >       
> Nit: Indentation.
>
> > +{
> > +    unsigned int level;
> > +    unsigned int target = page_order / PAGETABLE_ORDER;
> > +    pte_t *entry, *table, orig_pte;
> > +    int rc;
> > +    /*
> > +     * A mapping is removed only if the MFN is explicitly set to INVALID_MFN.
> > +     * Other MFNs that are considered invalid by mfn_valid() (e.g., MMIO)
> > +     * are still allowed.
> > +     */
> > +    bool removing_mapping = mfn_eq(mfn, INVALID_MFN);
> > +    DECLARE_OFFSETS(offsets, gfn_to_gaddr(gfn));
> > +
> > +    ASSERT(p2m_is_write_locked(p2m));
> > +
> > +    /*
> > +     * Check if the level target is valid: we only support
> > +     * 4K - 2M - 1G mapping.
> > +     */
> > +    ASSERT(target <= 2);
> > +
> > +    table = p2m_get_root_pointer(p2m, gfn);
> > +    if ( !table )
> > +        return -EINVAL;
> > +
> > +    for ( level = P2M_ROOT_LEVEL; level > target; level-- )
> > +    {
> > +        /*
> > +         * Don't try to allocate intermediate page table if the mapping
> > +         * is about to be removed.
> > +         */
> > +        rc = p2m_next_level(p2m, !removing_mapping,
> > +                            level, &table, offsets[level]);
> > +        if ( (rc == P2M_TABLE_MAP_NONE) || (rc == P2M_TABLE_MAP_NOMEM) )
> > +        {
> > +            rc = (rc == P2M_TABLE_MAP_NONE) ? -ENOENT : -ENOMEM;
> > +            /*
> > +             * We are here because p2m_next_level has failed to map
> > +             * the intermediate page table (e.g the table does not exist
> > +             * and they p2m tree is read-only).
> >       
> I thought I commented on this or something similar already: Calling the
> p2m tree "read-only" is imo misleading.
>     
I will change then "read-only" to "not allocatable".

> > It is a valid case
> > +             * when removing a mapping as it may not exist in the
> > +             * page table. In this case, just ignore it.
> >       
> I fear the "it" has no reference; aiui you mean "ignore the lookup failure",
> but the comment isn't worded to refer to that by "it".
>     
I will update the comment correspondingly.

> > +             */
> > +            rc = removing_mapping ? 0 : rc;
> > +            goto out;
> > +        }
> > +
> > +        if ( rc != P2M_TABLE_NORMAL )
> > +            break;
> > +    }
> > +
> > +    entry = table + offsets[level];
> > +
> > +    /*
> > +     * If we are here with level > target, we must be at a leaf node,
> > +     * and we need to break up the superpage.
> > +     */
> > +    if ( level > target )
> > +    {
> > +        panic("Shattering isn't implemented\n");
> > +    }
> > +
> > +    /*
> > +     * We should always be there with the correct level because all the
> > +     * intermediate tables have been installed if necessary.
> > +     */
> > +    ASSERT(level == target);
> > +
> > +    orig_pte = *entry;
> > +
> > +    if ( removing_mapping )
> > +        p2m_clean_pte(entry, p2m->clean_dcache);
> > +    else
> > +    {
> > +        pte_t pte = p2m_pte_from_mfn(mfn, t);
> > +
> > +        p2m_write_pte(entry, pte, p2m->clean_dcache);
> > +
> > +        p2m->max_mapped_gfn = gfn_max(p2m->max_mapped_gfn,
> > +                                      gfn_add(gfn, BIT(page_order, UL) - 1));
> > +        p2m->lowest_mapped_gfn = gfn_min(p2m->lowest_mapped_gfn, gfn);
> > +    }
> > +
> > +    p2m->need_flush = true;
> > +
> > +    /*
> > +     * Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
> > +     * is not ready for RISC-V support.
> > +     *
> > +     * When CONFIG_HAS_PASSTHROUGH=y, iommu_iotlb_flush() should be done
> > +     * here.
> > +     */
> > +#ifdef CONFIG_HAS_PASSTHROUGH
> > +#   error "add code to flush IOMMU TLB"
> > +#endif
> > +
> > +    rc = 0;
> > +
> > +    /*
> > +     * Free the entry only if the original pte was valid and the base
> > +     * is different (to avoid freeing when permission is changed).
> > +     *
> > +     * If previously MFN 0 was mapped and it is going to be removed
> > +     * and considering that during removing MFN 0 is used then `entry`
> > +     * and `new_entry` will be the same and p2m_free_subtree() won't be
> > +     * called. This case is handled explicitly.
> > +     */
> > +    if ( pte_is_valid(orig_pte) &&
> > +         (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) ||
> > +          (removing_mapping && mfn_eq(pte_get_mfn(*entry), _mfn(0)))) )
> > +        p2m_free_subtree(p2m, orig_pte, level);
> >       
> I continue to fail to understand why the MFN would matter here.
>     
My understanding is that if, for the same GFN, the MFN changes from MFN_1 to
MFN_2, then we need to update any references on the page referenced by
orig_pte to ensure the proper reference counter is maintained for the page
pointed to by MFN_1.
>  Isn't the
> need to free strictly tied to a VALID -> NOT VALID transition? A permission
> change simply retains the VALID state of an entry.
>     
It covers a case when removing happens and probably in this case we don't need
to check specifically for mfn(0) case "mfn_eq(pte_get_mfn(*entry), _mfn(0))",
but it would be enough to check that pte_is_valid(entry) instead:
  ...
  (removing_mapping && !pte_is_valid(entry)))) )

Or only check removing_mapping variable as `entry` would be invalided by the
code above anyway. So we will get:
+    if ( pte_is_valid(orig_pte) &&
+         (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) || removing_mapping) )
+        p2m_free_subtree(p2m, orig_pte, level);

Does it make sense now?

~ Oleksii