linux-kernel - Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for qualified platforms

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Date:   Wed, 1 Dec 2021 23:47:39 -0500
From:   Luming Yu <luming.yu@...il.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, paulmck@...nel.org,
        rui.zhang@...el.com, andi.kleen@...el.com,
        Len Brown <len.brown@...el.com>, tim.c.chen@...el.com
Subject: Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for qualified platforms

On Tue, Nov 16, 2021 at 11:18 PM Feng Tang <feng.tang@...el.com> wrote:
>
> There are cases that tsc clocksources are wrongly judged as unstable by
> clocksource watchdogs like hpet, acpi_pm or 'refined-jiffies'. While
> there is hardly a general reliable way to check the validity of a
> watchdog, and to protect the innocent tsc, Thomas Gleixner proposed [1]:
>
> "I'm inclined to lift that requirement when the CPU has:
>
>     1) X86_FEATURE_CONSTANT_TSC
>     2) X86_FEATURE_NONSTOP_TSC
>     3) X86_FEATURE_NONSTOP_TSC_S3
>     4) X86_FEATURE_TSC_ADJUST
>     5) At max. 4 sockets
>
>  After two decades of horrors we're finally at a point where TSC seems
>  to be halfway reliable and less abused by BIOS tinkerers. TSC_ADJUST
>  was really key as we can now detect even small modifications reliably
>  and the important point is that we can cure them as well (not pretty
>  but better than all other options)."
>
> As feature #3 X86_FEATURE_NONSTOP_TSC_S3 only exists on several generations
> of Atom processor, and is always coupled with X86_FEATURE_CONSTANT_TSC
> and X86_FEATURE_NONSTOP_TSC, skip checking it, and also be more defensive
> to use maxim of 2 sockets.
>
> The check is done inside tsc_init() before registering 'tsc-early' and
> 'tsc' clocksources, as there were cases that both of them had been
> wrongly judged as unreliable.
>
> For more background of tsc/watchdog, there is a good summary in [2]
>
> [1]. https://lore.kernel.org/lkml/87eekfk8bd.fsf@nanos.tec.linutronix.de/
> [2]. https://lore.kernel.org/lkml/87a6pimt1f.ffs@nanos.tec.linutronix.de/
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> ---
> Change log:
>
>   v3:
>     * rebased against 5.16-rc1
>     * refine commit log
>
>   v2:
>     * Directly skip watchdog check without messing flag
>       'tsc_clocksource_reliable' (Thomas)
>
>  arch/x86/kernel/tsc.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)

retry:

Reviewed-by: Luming Yu <luming.yu@...el.com>