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

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Message-ID: <xhsmhv7whhnjb.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 20 Nov 2024 17:34:32 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [RFC PATCH v3 08/15] sched/clock, x86: Make
 __sched_clock_stable forceful

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?

Ah, maybe not, I saw the below but that's actually the x86 specific stuff
and IIUC can only be builtin:

  kvm_init_platform()
  `\
    kvmclock_init()
    `\
      kvm_sched_clock_init()
      `\
        clear_sched_clock_stable()

There is however this:

  kvm_arch_vcpu_load()
  `\
    mark_tsc_unstable()

So plugging a VCPU might do that.