[PATCH v8 36/43] arm64: mm: Add support for folding PUDs at runtime

[Ard Biesheuvel]

On Mon, 30 Sept 2024 at 16:36, Ryan Roberts <ryan.roberts at arm.com> wrote:
>
> Hi Ard,
>
> On 14/02/2024 12:29, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb at kernel.org>
> >
> > In order to support LPA2 on 16k pages in a way that permits non-LPA2
> > systems to run the same kernel image, we have to be able to fall back to
> > at most 48 bits of virtual addressing.
> >
> > Falling back to 48 bits would result in a level 0 with only 2 entries,
> > which is suboptimal in terms of TLB utilization. So instead, let's fall
> > back to 47 bits in that case. This means we need to be able to fold PUDs
> > dynamically, similar to how we fold P4Ds for 48 bit virtual addressing
> > on LPA2 with 4k pages.
> >
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
>
> [...]
>
> >
> > +#define pud_index(addr)              (((addr) >> PUD_SHIFT) & (PTRS_PER_PUD - 1))
> > +
> > +static inline pud_t *p4d_to_folded_pud(p4d_t *p4dp, unsigned long addr)
> > +{
> > +     return (pud_t *)PTR_ALIGN_DOWN(p4dp, PAGE_SIZE) + pud_index(addr);
> > +}
> > +
>
> I wonder if you could explain what this function (and its equivalents at other
> levels) is doing? Why isn't it just returning p4dp cast to a (pud_t *)?
>

Because the p4dp index is derived from a different part of the VA, so
it points into the right page but at the wrong entry.

> I'm working on a prototype for boot-time page size selection. For this, I'm
> compile-time enabling all levels, then run-time folding the ones I don't need,
> based on the selected page size and VA size.
>

Nice!

> I'm trying to reuse your run-time folding code, but I have a case where this
> function is broken as written. Replacing with "return (pud_t *)p4dp;" resolves
> the problem; If VA_BITS=48 and pagesize=64K, the pgd has 64 entries. p4dp is
> pointing to the correct entry in the pgd already, but this code aligns back to
> the start of the page, then adds pud_index(), which is wrong because
> PTRS_PER_PUD != PTRS_PER_PGDIR. (In my case, these 2 macros are actually
> boot-time selected values rather than compile-time constants).
>
> I think your code is probably correct and working around PTRS_PER_PXD being
> compile-time constants for the non-folded case, but I can't quite convince myself.
>

Is your p4dp pointing to the correct descriptor because you changed
the runtime behavior of p4d_index() perhaps?