linux-kernel - Re: [PATCH 10/10] arm64: mm: Re-implement the __flush_tlb_range

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Message-ID: <CAHk-=wijmAO=-dWc8HUMdUbkdGqgNtiO6mntAcTekWc5qN3YjQ@mail.gmail.com>
Date: Fri, 11 Jul 2025 11:16:32 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
	Ryan Roberts <ryan.roberts@....com>, Mark Rutland <mark.rutland@....com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH 10/10] arm64: mm: Re-implement the __flush_tlb_range_op
 macro in C

On Fri, 11 Jul 2025 at 09:18, Will Deacon <will@...nel.org> wrote:
>
> The __flush_tlb_range_op() macro is horrible and has been a previous
> source of bugs thanks to multiple expansions of its arguments (see
> commit f7edb07ad7c6 ("Fix mmu notifiers for range-based invalidates")).
>
> Rewrite the thing in C.

So I do think this is better than the old case, but I think you could
go one step further...

> +static __always_inline void __flush_tlb_range_op(const enum tlbi_op op,
> +                                                u64 start, size_t pages,
> +                                                u64 stride, u16 asid,
> +                                                u32 level, bool lpa2)

If you replaced that "enum tlbi_op op" with two function pointers
instead (for "the invalidate range" and "invalidate one" cases
respectively), I think you could make this code much more obvious.

And exactly like how you depend on that 'op' value being
constant-folded because all the different levels are inline functions,
the same thing ends up happening with function pointers where inlining
will result in a constant function pointer becoming just a static call
(and in turn inlined as well).

And then the actual *callers* would use the "look up op" thing, but I
suspect that in many cases those could then be in turn also simplified
to not use that op-number at all, but just end up using the op-name.

I didn't try to actually create that series - and I think you'd want
to do it in multiple steps just to make each individual step small and
obvious - but I think it would end up looking nicer.

            Linus