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

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

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On 9/26/25 9:07 AM, Jan Beulich wrote:
> On 25.09.2025 22:08, Oleksii Kurochko wrote:
>> On 9/20/25 1:36 AM, Jan Beulich wrote:
>>> On 17.09.2025 23:55, Oleksii Kurochko wrote:
>>>> +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?
> Yes and no. I understand what you're after, but that doesn't make the use of
> P2M_PAGETABLE_ENTRIES() (with an arbitrary level as argument) correct. This
> calculation wants doing by solely using properties of the top level.

Got it, thanks. Then I will use solely properties of the top level.

>>>> +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".
> That'll be only marginally better: What's "allocatable"? Why not something
> like "... does not exist and none should be allocated"? Or maybe simply
> omit this part of the comment?

Agree, "allocatable" could be also confusing. Perhaps, just omitting will
be fine.

>
>>>> +    /*
>>>> +     * 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 fromMFN_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 byMFN_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?
> Not really, sorry. Imo the complicated condition indicates that something is
> wrong (or at least inefficient) here.

Then, in the case of aVALID -> VALID transition, where the MFN is changed for the
same PTE, should something be done with the old MFN (e.g., calling|p2m_put_page()|
for it), or can freeing the old MFN be delayed until|domain_relinquish_resources() |is called? If so, wouldn’t that lead to a situation where many old MFNs accumulate
and cannot be re-used until|domain_relinquish_resources()| (or another function that
explicitly frees pages) is invoked?

If we only need to care about theVALID -> NOT VALID transition, doesn’t that mean
|p2m_free_subtree()| should be called only when a removal actually occurs?

~ Oleksii


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