Re: [PATCH v10 12/21] gpu: nova-core: mm: Add unified page table entry wrapper enums

[Eliot Courtney]

On Tue Apr 7, 2026 at 6:55 AM JST, Joel Fernandes wrote:
>>> +    /// Compute upper bound on page table pages needed for `num_virt_pages`.
>>> +    ///
>>> +    /// Walks from PTE level up through PDE levels, accumulating the tree.
>>> +    pub(crate) fn pt_pages_upper_bound(&self, num_virt_pages: usize) -> usize {
>>> +        let mut total = 0;
>>> +
>>> +        // PTE pages at the leaf level.
>>> +        let pte_epp = self.entries_per_page(self.pte_level());
>>> +        let mut pages_at_level = num_virt_pages.div_ceil(pte_epp);
>>> +        total += pages_at_level;
>>> +
>>> +        // Walk PDE levels bottom-up (reverse of pde_levels()).
>>> +        for &level in self.pde_levels().iter().rev() {
>>> +            let epp = self.entries_per_page(level);
>>> +
>>> +            // How many pages at this level do we need to point to
>>> +            // the previous pages_at_level?
>>> +            pages_at_level = pages_at_level.div_ceil(epp);
>>> +            total += pages_at_level;
>>> +        }
>>> +
>>> +        total
>>> +    }
>>> +}
>>> +
>> 
>> We have a lot of matches on the MMU version here (and below in Pte, Pde,
>> DualPde). What about making MmuVersion into a trait (e.g. Mmu) with
>> associated types for Pte, Pde, DualPde which can implement traits
>> defining their common operations too?
>
> I coded this up and it did not look pretty, there's not much LOC savings and the
> code becomes harder to read because of parametrization of several functions. Also:

Thanks for looking into it. Sorry to be a bother, but would you have a
branch around with the code? I'm curious what didn't look good about it.

>> Then you can parameterise Vmm/PtWalk on this type.
>
> The match still to be done somewhere, so you end up matching on chipset to call
> the correct parametrized functions versus just passing in the parameter or
> chipset down, in some cases.
>
> For now I am inclined to leave it as is. Also there's a Rust pitfall we all
> learnt during the turing and other patch reviews, sometimes doing a bunch of
> matches is good especially if the number of variants are expected to be fixed
> (in the mm case, version 2 and version 3). Traits have some disadvantages too,
> example dyn traits have to heap-allocated, parametrizing can increase code size
> (due to monomorphization) etc.

Yeah, it's just this is a lot of matches in a lot of places. And we have
ver2 / ver3 specific code leaking into the general pagetable.rs file. So
it would be really nice if we could find a way to improve this specific
aspect. We can reduce the match to happening in just one file. You can
avoid heap allocation if you would like by making Vmm an enum,
for example, and doing the match based dispatch there at the top of the
API tree, rather than at the bottom where it fans out into a lot more
locations.

>
> thanks,
>
> --
> Joel Fernandes