Re: [PATCH v3 00/12] mm/gup: Unify hugetlb, part 2

[Christophe Leroy]

Le 10/04/2024 à 21:58, Peter Xu a écrit :
>>
>> e500 has two modes: 32 bits and 64 bits.
>>
>> For 32 bits:
>>
>> 8xx is the only one handling it through HW-assisted pagetable walk hence
>> requiring a 2-level whatever the pagesize is.
> 
> Hmm I think maybe finally I get it..
> 
> I think the confusion came from when I saw there's always such level-2
> table described in Figure 8-5 of the manual:
> 
> https://www.nxp.com/docs/en/reference-manual/MPC860UM.pdf

Yes indeed that figure is confusing.

Table 8-1 gives a pretty good idea of what is required. We only use 
MD_CTR[TWAM] = 1

> 
> So I suppose you meant for 8M, the PowerPC 8xx system hardware will be
> aware of such 8M pgtable (from level-1's entry, where it has bit 28-29 set
> 011b), then it won't ever read anything starting from "Level-2 Descriptor
> 1" (but only read the only entry "Level-2 Descriptor 0"), so fundamentally
> hugepd format must look like such for 8xx?
> 
> But then perhaps it's still compatible with cont-pte because the rest
> entries (pte index 1+) will simply be ignored by the hardware?

Yes, still compatible with CONT-PTE allthough things become tricky 
because you need two page tables to get the full 8M so that's a kind of 
cont-PMD down to PTE level, as you can see in my RFC series.

> 
>>
>> On e500 it is all software so pages 2M and larger should be cont-PGD (by
>> the way I'm a bit puzzled that on arches that have only 2 levels, ie PGD
>> and PTE, the PGD entries are populated by a function called PMD_populate()).
> 
> Yeah.. I am also wondering whether pgd_populate() could also work there
> (perhaps with some trivial changes, or maybe not even needed..), as when
> p4d/pud/pmd levels are missing, linux should just do something like an
> enforced cast from pgd_t* -> pmd_t* in this case.
> 
> I think currently they're already not pgd, as __find_linux_pte() already
> skipped pgd unconditionally:
> 
> 	pgdp = pgdir + pgd_index(ea);
> 	p4dp = p4d_offset(pgdp, ea);
> 

Yes that's what is confusing, some parts of code considers we have only 
a PGD and a PT while other parts consider we have only a PMD and a PT

>>
>> Current situation for 8xx is illustrated here:
>> https://github.com/linuxppc/wiki/wiki/Huge-pages#8xx
>>
>> I also tried to better illustrate e500/32 here:
>> https://github.com/linuxppc/wiki/wiki/Huge-pages#e500
>>
>> For 64 bits:
>> We have PTE/PMD/PUD/PGD, no P4D
>>
>> See arch/powerpc/include/asm/nohash/64/pgtable-4k.h
> 
> We don't have anything that is above pud in this category, right?  That's
> what I read from your wiki (and thanks for providing that in the first
> place; helps a lot for me to understand how it works on PowerPC).

Yes thanks to Michael and Aneesh who initiated that Wiki page.

> 
> I want to make sure if I can move on without caring on p4d/pgd leafs like
> what we do right now, even after if we can remove hugepd for good, in this
> case since p4d always missing, then it's about whether "pud|pmd|pte_leaf()"
> can also cover the pgd ones when that day comes, iiuc.

I guess so but I'd like Aneesh and/or Michael to confirm as I'm not an 
expert on PPC64.

Christophe