linux-kernel - Re: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment

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Date:   Wed, 19 Sep 2018 13:51:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     aneesh.kumar@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        npiggin@...il.com, linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux@...linux.org.uk,
        heiko.carstens@...ibm.com
Subject: Re: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment

On Fri, Sep 14, 2018 at 05:48:57PM +0100, Will Deacon wrote:

> > + *  - mmu_gather::fullmm
> > + *
> > + *    A flag set by tlb_gather_mmu() to indicate we're going to free
> > + *    the entire mm; this allows a number of optimizations.
> > + *
> > + *    XXX list optimizations
> 
> On arm64, we can elide the invalidation altogether because we won't
> re-allocate the ASID. We also have an invalidate-by-ASID (mm) instruction,
> which we could use if we needed to.

Right, but I was also struggling to put into words the normal fullmm
case.

I now ended up with:

--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -82,7 +82,11 @@
  *    A flag set by tlb_gather_mmu() to indicate we're going to free
  *    the entire mm; this allows a number of optimizations.
  *
- *    XXX list optimizations
+ *    - We can ignore tlb_{start,end}_vma(); because we don't
+ *      care about ranges. Everything will be shot down.
+ *
+ *    - (RISC) architectures that use ASIDs can cycle to a new ASID
+ *      and delay the invalidation until ASID space runs out.
  *
  *  - mmu_gather::need_flush_all
  *

Does that about cover things; or do we need more?

> > + *
> > + *  - mmu_gather::need_flush_all
> > + *
> > + *    A flag that can be set by the arch code if it wants to force
> > + *    flush the entire TLB irrespective of the range. For instance
> > + *    x86-PAE needs this when changing top-level entries.
> > + *
> > + * And requires the architecture to provide and implement tlb_flush().
> > + *
> > + * tlb_flush() may, in addition to the above mentioned mmu_gather fields, make
> > + * use of:
> > + *
> > + *  - mmu_gather::start / mmu_gather::end
> > + *
> > + *    which (when !need_flush_all; fullmm will have start = end = ~0UL) provides
> > + *    the range that needs to be flushed to cover the pages to be freed.
> 
> I don't understand the mention of need_flush_all here -- I didn't think it
> was used by the core code at all.

The core does indeed not use that flag; but if the architecture set
that, the range is still ignored.

Can you suggest clearer wording?