Re: [PATCH v1 1/3] xen/riscv: implement software page table walking

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.01.2025 18:22, Oleksii Kurochko wrote:
> 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@xxxxxxxxx>
>>>>> ---
>>>>>    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.

Imo the most natural thing for a page walking function would be to return the
leaf PTE (or the leaf-most not-present [or otherwise "no-access"] one). That
would provide (almost) all possible information to the caller. "Almost"
because depending on how page walk works, permissions may combine across page
table levels. Yet then (see also the "no-access" above) this would also
require further input, to specify the context for which the translation is
being seeked. For example, the intention to write may want to yield no valid
PTE when there are present ones down to the leaf, but effective permissions
say "read-only".

Jan