On 1/27/25 1:57 PM, Jan Beulich wrote:
> On 27.01.2025 13:29, Oleksii Kurochko wrote:
>> On 1/27/25 11:06 AM, Jan Beulich wrote:
>>> On 20.01.2025 17:54, Oleksii Kurochko wrote:
>>>> RISC-V doesn't have hardware feature to ask MMU to translate
>>>> virtual address to physical address ( like Arm has, for example ),
>>>> so software page table walking in implemented.
>>>>
>>>> Signed-off-by: Oleksii Kurochko<oleksii.kurochko@gmail.com>
>>>> ---
>>>>    xen/arch/riscv/include/asm/mm.h |  2 ++
>>>>    xen/arch/riscv/pt.c             | 56 +++++++++++++++++++++++++++++++++
>>>>    2 files changed, 58 insertions(+)
>>>>
>>>> diff --git a/xen/arch/riscv/include/asm/mm.h b/xen/arch/riscv/include/asm/mm.h
>>>> index 292aa48fc1..d46018c132 100644
>>>> --- a/xen/arch/riscv/include/asm/mm.h
>>>> +++ b/xen/arch/riscv/include/asm/mm.h
>>>> @@ -15,6 +15,8 @@
>>>>    
>>>>    extern vaddr_t directmap_virt_start;
>>>>    
>>>> +paddr_t pt_walk(vaddr_t va);
>>> In the longer run, is returning just the PA really going to be sufficient?
>>> If not, perhaps say a word on the limitation in the description.
>> In the long run, this function's prototype looks like|paddr_t pt_walk(vaddr_t root, vaddr_t va, bool is_xen)| [1]. However, I'm not sure if it will stay that way,
>> as I think|is_xen| could be skipped, since using|map_table()| should be sufficient (as it now considers|system_state|) and I'm not really sure if I need root argument
>> as initial goal was to use this function for debug only purposes and I've never used it for guest page table (stage-1) walking.
>> Anyway, yes, it is still returning a physical address, and that seems enough to me.
>>
>> Could you share your thoughts on what I should take into account for returning value, probably, I am missing something really useful?
> Often you care about the permissions as well. Sometimes it may even be relevant
> to know the (super-)page size of the mapping.

Perhaps it would be better to change the prototype to:
   bool pt_walk(vaddr_t va, mfn_t *ret_pa);
or even
   void pt_walk(vaddr_t va, mfn_t *ret_pa);
   In this case,|ret_pa = INVALID_MFN| could serve as a signal that|pt_walk()| failed.
If there's a need to return permissions or (super-)page size in the future, another argument could be added.

What do you think? Would this approach be better?

I am also considering returning a structure containing the|mfn| (or|paddr_t|) and adding other properties (such as permissions or
page size) as needed in the future. Both solutions seem more or less equivalent.

~ Oleksii