linux-kernel - Re: [RFC PATCH v3 08/15] sched/clock, x86: Make __sched_clock

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Message-ID: <20241121110242.GD24774@noisy.programming.kicks-ass.net>
Date: Thu, 21 Nov 2024 12:02:42 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, bpf@...r.kernel.org,
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Subject: Re: [RFC PATCH v3 08/15] sched/clock, x86: Make __sched_clock_stable
 forceful

On Wed, Nov 20, 2024 at 05:34:32PM +0100, Valentin Schneider wrote:
> On 20/11/24 15:59, Peter Zijlstra wrote:
> > On Tue, Nov 19, 2024 at 04:34:55PM +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.
> >> 
> >> __sched_clock_stable is used in .noinstr code, and can be modified at
> >> runtime (e.g. KVM module loading). Suppressing the text_poke_sync() IPI has
> >
> > Wait, what !? loading KVM causes the TSC to be marked unstable?
> 

> There is however this:
> 
>   kvm_arch_vcpu_load()
>   `\
>     mark_tsc_unstable()
> 
> So plugging a VCPU might do that.

Right, but that only happens if it observes the TSC doing dodgy, so
that's deserved and shouldn't happen on hardware from this decade, and
possibly the one before that.