linux-kernel - Re: [PATCH 4/7] arm64: add 'runtime constant' support

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Date: Tue, 11 Jun 2024 21:22:27 +0100
From: Mark Rutland <mark.rutland@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 4/7] arm64: add 'runtime constant' support

On Tue, Jun 11, 2024 at 11:59:21AM -0700, Linus Torvalds wrote:
> On Tue, 11 Jun 2024 at 10:59, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > So I'll look at doing this for x86 and see how it works.
> 
> Oh - and when I started looking at it, I immediately remembered why I
> didn't want to use alternatives originally.
> 
> The alternatives are finalized much too early for this. By the time
> the dcache code works, the alternatives have already been applied.
> 
> I guess all the arm64 alternative callbacks are basically finalized
> very early, basically when the CPU models etc have been setup.

On arm64 we have early ("boot") and late ("system-wide") alternatives.
We apply the system-wide alternatives in apply_alternatives_all(), a few
callees deep under smp_cpus_done(), after secondary CPUs are brought up,
since that has to handle mismatched features in big.LITTLE systems.

I had assumed that we could use late/system-wide alternatives here, since
those get applied after vfs_caches_init_early(), but maybe that's too
late?

> We could do a "late alternatives", I guess, but now it's even more
> infrastructure just for the constants.

Fair enough; thanks for taking a look.

Mark.