[RFC PATCH 3/3] powerpc/64s/radix: optimise TLB flush with precise TLB ranges in mmu_gather

[Nicholas Piggin]

On Tue, 12 Jun 2018 16:26:33 -0700
Linus Torvalds <torvalds at linux-foundation.org> wrote:

> On Tue, Jun 12, 2018 at 4:09 PM Nicholas Piggin <npiggin at gmail.com> wrote:
> >
> > Sorry I mean Intel needs the existing behaviour of range flush expanded
> > to cover page table pages.... right?  
> 
> Right.  Intel depends on the current thing, ie if a page table
> *itself* is freed, we will will need to do a flush, but it's the exact
> same flush as if there had been a regular page there.
> 
> That's already handled by (for example) pud_free_tlb() doing the
> __tlb_adjust_range().

Agreed.

> 
> Again, I may be missing entirely what you're talking about, because it
> feels like we're talking across each other.
> 
> My argument is that your new patches in (2-3 in the series - patch #1
> looks ok) seem to be fundamentally specific to things that have a
> *different* tlb invalidation for the directory entries than for the
> leaf entries.

Yes I think I confused myself a bit. You're right these patches are
only useful if there is no page structure cache, or if it's managed
separately from TLB invalidation.

> 
> But that's not what at least x86 has, and not what the generic code has done.
> 
> I think it might be fine to introduce a few new helpers that end up
> being no-ops for the traditional cases.
> 
> I just don't think it makes sense to maintain a set of range values
> that then aren't actually used in the general case.

Sure, I'll make it optional. That would probably give a better result
for powerpc too because it doesn't need to maintain two ranges either.

Thanks,
Nick