Message-ID: <20241120145649.GJ19989@noisy.programming.kicks-ass.net>
Date: Wed, 20 Nov 2024 15:56:49 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org,
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Subject: Re: [RFC PATCH v3 06/15] jump_label: Add forceful jump label type

On Tue, Nov 19, 2024 at 03:39:02PM -0800, Josh Poimboeuf wrote:
> On Tue, Nov 19, 2024 at 04:34:53PM +0100, Valentin Schneider wrote:
> > Later commits will cause objtool to warn about non __ro_after_init static
> > keys being used in .noinstr sections in order to safely defer instruction
> > patching IPIs targeted at NOHZ_FULL CPUs.
> 
> Don't we need similar checking for static calls?
> 
> > Two such keys currently exist: mds_idle_clear and __sched_clock_stable,
> > which can both be modified at runtime.
> 
> Not sure if feasible, but it sure would be a lot simpler to just make
> "no noinstr patching" a hard rule and then convert the above keys (or at
> least their noinstr-specific usage) to regular branches.

That'll be a bit of a mess. Also, then we're adding overhead/cost for
all people for the benefit of this fringe case (NOHZ_FULL). Not a
desirable trade-off IMO.

So I do think the proposed solution (+- naming, I like your naming
proposal better) is the better one.

But I think we can make the fall-back safer, we can simply force the IPI
when we poke at noinstr code -- then NOHZ_FULL gets to keep the pieces,
but at least we don't violate any correctness constraints.

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