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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 1/27/25 6:22 PM, 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?

We have to return mfn_t or paddr_t as pt_walk() is used in vmap_to_mfn().
~ Oleksii
> 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
>