Re: [PATCH v2 17/17] xen/riscv: add support of page lookup by GFN

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

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On 7/2/25 1:44 PM, Jan Beulich wrote:
> On 10.06.2025 15:05, Oleksii Kurochko wrote:
>> Introduce helper functions for safely querying the P2M (physical-to-machine)
>> mapping:
>>   - add p2m_read_lock(), p2m_read_unlock(), and p2m_is_locked() for managing
>>     P2M lock state.
>>   - Implement p2m_get_entry() to retrieve mapping details for a given GFN,
>>     including MFN, page order, and validity.
>>   - Add p2m_lookup() to encapsulate read-locked MFN retrieval.
>>   - Introduce p2m_get_page_from_gfn() to convert a GFN into a page_info
>>     pointer, acquiring a reference to the page if valid.
>>
>> Implementations are based on Arm's functions with some minor modifications:
>> - p2m_get_entry():
>>    - Reverse traversal of page tables, as RISC-V uses the opposite order
>>      compared to Arm.
>>    - Removed the return of p2m_access_t from p2m_get_entry() since
>>      mem_access_settings is not introduced for RISC-V.
> Didn't I see uses of p2m_access in earlier patches? If you don't mean to have
> that, then please consistently {every,no}where.

Yes, it was used. I think it would be better just usage of p2m_access from earlier
patches.

>>    - Updated BUILD_BUG_ON() to check using the level 0 mask, which corresponds
>>      to Arm's THIRD_MASK.
>>    - Replaced open-coded bit shifts with the BIT() macro.
>>    - Other minor changes, such as using RISC-V-specific functions to validate
>>      P2M PTEs, and replacing Arm-specific GUEST_* macros with their RISC-V
>>      equivalents.
>> - p2m_get_page_from_gfn():
>>    - Removed p2m_is_foreign() and related logic, as this functionality is not
>>      implemented for RISC-V.
> Yet I expect you'll need this, sooner or later.

Then I'll add correspondent code in this patch.

>> --- a/xen/arch/riscv/p2m.c
>> +++ b/xen/arch/riscv/p2m.c
>> @@ -1055,3 +1055,134 @@ int guest_physmap_add_entry(struct domain *d,
>>   {
>>       return p2m_insert_mapping(d, gfn, (1 << page_order), mfn, t);
>>   }
>> +
>> +/*
>> + * Get the details of a given gfn.
>> + *
>> + * If the entry is present, the associated MFN will be returned and the
>> + * access and type filled up. The page_order will correspond to the
> You removed p2m_access_t * from the parameters; you need to also update
> the comment then accordingly.
>
>> + * order of the mapping in the page table (i.e it could be a superpage).
>> + *
>> + * If the entry is not present, INVALID_MFN will be returned and the
>> + * page_order will be set according to the order of the invalid range.
>> + *
>> + * valid will contain the value of bit[0] (e.g valid bit) of the
>> + * entry.
>> + */
>> +static mfn_t p2m_get_entry(struct p2m_domain *p2m, gfn_t gfn,
>> +                           p2m_type_t *t,
>> +                           unsigned int *page_order,
>> +                           bool *valid)
>> +{
>> +    paddr_t addr = gfn_to_gaddr(gfn);
>> +    unsigned int level = 0;
>> +    pte_t entry, *table;
>> +    int rc;
>> +    mfn_t mfn = INVALID_MFN;
>> +    p2m_type_t _t;
> Please no local variables with leading underscores. In x86 we commonly
> name such variables p2mt.
>
>> +    DECLARE_OFFSETS(offsets, addr);
> This is the sole use of "addr". Is such a local variable really worth having?

Not really, I'll drop it.

>> +    ASSERT(p2m_is_locked(p2m));
>> +    BUILD_BUG_ON(XEN_PT_LEVEL_MAP_MASK(0) != PAGE_MASK);
>> +
>> +    /* Allow t to be NULL */
>> +    t = t ?: &_t;
>> +
>> +    *t = p2m_invalid;
>> +
>> +    if ( valid )
>> +        *valid = false;
>> +
>> +    /* XXX: Check if the mapping is lower than the mapped gfn */
>> +
>> +    /* This gfn is higher than the highest the p2m map currently holds */
>> +    if ( gfn_x(gfn) > gfn_x(p2m->max_mapped_gfn) )
>> +    {
>> +        for ( level = P2M_ROOT_LEVEL; level ; level-- )
> Nit: Stray blank before the 2nd semicolon. (Again at least once below.)
>
>> +            if ( (gfn_x(gfn) & (XEN_PT_LEVEL_MASK(level) >> PAGE_SHIFT)) >
>> +                 gfn_x(p2m->max_mapped_gfn) )
>> +                break;
>> +
>> +        goto out;
>> +    }
>> +
>> +    table = p2m_get_root_pointer(p2m, gfn);
>> +
>> +    /*
>> +     * the table should always be non-NULL because the gfn is below
>> +     * p2m->max_mapped_gfn and the root table pages are always present.
>> +     */
>> +    if ( !table )
>> +    {
>> +        ASSERT_UNREACHABLE();
>> +        level = P2M_ROOT_LEVEL;
>> +        goto out;
>> +    }
>> +
>> +    for ( level = P2M_ROOT_LEVEL; level ; level-- )
>> +    {
>> +        rc = p2m_next_level(p2m, true, level, &table, offsets[level]);
>> +        if ( (rc == GUEST_TABLE_MAP_NONE) && (rc != GUEST_TABLE_MAP_NOMEM) )
> This condition looks odd. As written the rhs of the && is redundant.

And it is wrong. It should:
  if ( (rc == P2M_TABLE_MAP_NONE) || (rc == P2M_TABLE_MAP_NOMEM) )

>> +            goto out_unmap;
>> +        else if ( rc != GUEST_TABLE_NORMAL )
> As before, no real need for "else" in such cases.
>
>> +            break;
>> +    }
>> +
>> +    entry = table[offsets[level]];
>> +
>> +    if ( p2me_is_valid(p2m, entry) )
>> +    {
>> +        *t = p2m_type_radix_get(p2m, entry);
> If the incoming argument is NULL, the somewhat expensive radix tree lookup
> is unnecessary here.

Good point.

>> +        mfn = pte_get_mfn(entry);
>> +        /*
>> +         * The entry may point to a superpage. Find the MFN associated
>> +         * to the GFN.
>> +         */
>> +        mfn = mfn_add(mfn,
>> +                      gfn_x(gfn) & (BIT(XEN_PT_LEVEL_ORDER(level), UL) - 1));
>> +
>> +        if ( valid )
>> +            *valid = pte_is_valid(entry);
> Interesting. Why not the P2M counterpart of the function? Yes, the comment
> ahead of the function says so, but I don't see why the valid bit suddenly
> is relevant here (besides the P2M type).

This valid variable is expected to be used in the caller (something what Arm does here
https://gitlab.com/xen-project/xen/-/blob/staging/xen/arch/arm/p2m.c#L375) to check if
it is needed to do flush_page_to_ram(), so if the the valid bit in PTE was set to 0
then it means nothing should be flushed as entry wasn't used as it marked invalid.

>> +    }
>> +
>> +out_unmap:
>> +    unmap_domain_page(table);
>> +
>> +out:
> Nit: Style (bot labels).
>
>> +    if ( page_order )
>> +        *page_order = XEN_PT_LEVEL_ORDER(level);
>> +
>> +    return mfn;
>> +}
>> +
>> +static mfn_t p2m_lookup(struct domain *d, gfn_t gfn, p2m_type_t *t)
> pointer-to-const for the 1st arg? But again more likely struct p2m_domain *
> anyway?

|struct p2_domain| would be better, but I’m not really sure that a
pointer-to-const can be used here. I expect that, in that case,
|p2m_read_lock()| would also need to accept a pointer-to-const, and since it
calls|read_lock()| internally, that could be a problem because|read_lock() |accepts a|rwlock_t *l|.

>> +{
>> +    mfn_t mfn;
>> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
>> +
>> +    p2m_read_lock(p2m);
>> +    mfn = p2m_get_entry(p2m, gfn, t, NULL, NULL);
>> +    p2m_read_unlock(p2m);
>> +
>> +    return mfn;
>> +}
>> +
>> +struct page_info *p2m_get_page_from_gfn(struct domain *d, gfn_t gfn,
> Same here - likely you mean struct p2m_domain * instead.
>
>> +                                        p2m_type_t *t)
>> +{
>> +    p2m_type_t p2mt = {0};
> Why a compound initializer for something that isn't a compound object?
> And why plain 0 for something that is an enumerated type?

Agree, it should be a compound initializer. I'll drop it.

>
>> +    struct page_info *page;
>> +
>> +    mfn_t mfn = p2m_lookup(d, gfn, &p2mt);
>> +
>> +    if ( t )
>> +        *t = p2mt;
> What's wrong with passing t directly to p2m_lookup()?

It was needed before when the code of p2m_get_page_from_gfn() looked like:
   struct page_info *p2m_get_page_from_gfn(struct domain *d, gfn_t gfn,
                                           p2m_type_t *t)
   {
       struct page_info *page;
       p2m_type_t p2mt;
       mfn_t mfn = p2m_lookup(d, gfn, &p2mt);
   
       if ( t )
           *t = p2mt;
   
       if ( !p2m_is_any_ram(p2mt) )
           return NULL;
So it was needed to make sure that p2m_is_any_ram(*t) doesn't try to dereference
a NULL pointer.

Even with the current one implementation the similar issue could be with if use
*t instead of p2mt:
   struct page_info *p2m_get_page_from_gfn(struct p2m_domain *p2m, gfn_t gfn,
                                           p2m_type_t *t)
   {
       ...
       if ( p2m_is_foreign(p2mt) )
       {
           struct domain *fdom = page_get_owner_and_reference(page);

And the second reason it is because of p2m_get_entry() (which is used inside
p2m_lookup()) could return for `t` a pointer to local variable:
   static mfn_t p2m_get_entry(struct p2m_domain *p2m, gfn_t gfn,
                              p2m_type_t *t,
                              unsigned int *page_order,
                              bool *valid)
   {
       ...
       p2m_type_t p2mt;
       ...
       /* Allow t to be NULL */
       t = t ?: &p2mt;
       ...

What looks wrong. I will remove this part of the code and pass
`t` directly to p2m_lookup().

And after p2m_lookup() call will just check if t argument is NULL then init
it with p2mt:
    struct page_info *p2m_get_page_from_gfn(struct p2m_domain *p2m, gfn_t gfn,
                                            p2m_type_t *t)
    {
        struct page_info *page;
        p2m_type_t p2mt = p2m_invalid;
        mfn_t mfn = p2m_lookup(p2m, gfn, t);
    
        if ( !mfn_valid(mfn) )
            return NULL;
    
        if ( !t )
            p2mt = *t;
    
        ...
    }

Thanks.

~ Oleksii

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