Re: [PATCH v4 17/18] xen/riscv: add support of page lookup by GFN

[Jan Beulich <jbeulich@suse.com>]

On 30.09.2025 17:37, Oleksii Kurochko wrote:
> On 9/22/25 10:46 PM, Jan Beulich wrote:
>> On 17.09.2025 23:55, Oleksii Kurochko wrote:
>>> +        if ( rc != P2M_TABLE_NORMAL )
>>> +            break;
>>> +    }
>>> +
>>> +    entry = table[offsets[level]];
>>> +
>>> +    if ( pte_is_valid(entry) )
>>> +    {
>>> +        if ( t )
>>> +            *t = p2m_get_type(entry);
>>> +
>>> +        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(P2M_LEVEL_ORDER(level), UL) - 1));
>> May want to assert that the respective bits of "mfn" are actually clear
>> before this calculation.
> 
> ASSERT(!(mfn & (BIT(P2M_LEVEL_ORDER(level), UL) - 1)));
> Do you mean something like that?

Yes.

> I am not 100% sure that there is really need for that as page-fault exception
> is raised if the PA is insufficienlty aligned:
>   Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv39 supports
>   2 MiB megapages and 1 GiB gigapages, each of which must be virtually and
>   physically aligned to a boundary equal to its size. A page-fault exception is
>   raised if the physical address is insufficiently aligned.

But that would be raised only when a page walk encounters such a PTE. You may
be altering a PTE here which never was involved in a page walk, though.

Jan