Re: [PATCH RFC 03/11] x86/mm: Add lookup_pgtable_in_pgd()

[Brendan Jackman <jackmanb@google.com>]

On Thu, Mar 13, 2025 at 10:09:21PM +0000, Yosry Ahmed wrote:
> On Thu, Mar 13, 2025 at 06:11:22PM +0000, Brendan Jackman wrote:
> > This is the same thing as lookup_address_in_pgd(), but it returns the
> > pagetable unconditionally instead of returning NULL when the pagetable
> > is none. This will be used for looking up and modifying pages that are
> > *_none() in order to map memory into the ASI restricted address space.
> > 
> > For a [PATCH], if this logic is needed, the surrounding code should
> > probably first be somewhat refactored. It now looks pretty repetitive,
> > and it's confusing that lookup_address_in_pgd() returns NULL when
> > pmd_none() but note when pte_none(). For now here's something that
> > works.
> 
> My first instinct reading this is that lookup_address_in_pgd() should be
> calling lookup_pgtable_in_pgd(), but I didn't look too closely.

Yeah. That outer function would get a "generic" PTE pointer isntead of
a strongly-typed p4d_t/pud_t/etc. So we either need to encode
assumptions that all the page tables have the same structure at
different levels for the bits we care about, or we need to have a
switch(*level) and then be careful about pgtable_l5_enabled(). I
think the former is fine but it needs a bit of care and attention to
ensure we don't miss anything and avoid creating
confusion/antipatterns in the code.

And perhaps more importantly, lookup_adress_in_pgd_attr() sets *nx and
*rw based on the level above the entry it returns.  E.g. when it
returns a pte_t* it sets *nx pased on pmd_flags(). I haven't looked
into why this is.

So yeah overall it needs a bit of research and most likely needs a
couple of prep patches. Hopefully it's possible to do it in a way that
leaves the existing code in a clearer state.

Anyway, I was originally planning not to have asi_map()/asi_unmap() in
asi.c at all, and instead just kinda make set_memory.c natively aware
of ASI somehow. At that point I think this code is probably gonna look
a bit different. That's something I ran out of time for and had to
drop from the scope of this RFC. It's definitely not ideal in this
series that e.g. page_alloc.c, asi.c, and set_memory.c are all
implicitly coupled to one another (i.e. they are all colluding to
ensure asi_[un]map() never has to allocate). Maybe I should've called
this out as a TODO on the cover letter actually.