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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     paulmck@kernel.org, Feng Tang <feng.tang@intel.com>,
        kernel test robot <oliver.sang@intel.com>,
        0day robot <lkp@intel.com>,
        John Stultz <john.stultz@linaro.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mark Rutland <Mark.Rutland@arm.com>,
        Marc Zyngier <maz@kernel.org>, Andi Kleen <ak@linux.intel.com>,
        Xing Zhengjun <zhengjun.xing@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        kernel-team@fb.com, neeraju@codeaurora.org,
        zhengjun.xing@intel.com, x86@kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [clocksource]  8c30ace35d: WARNING:at_kernel/time/clocksource.c:#clocksource_watchdog
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Date:   Wed, 28 Apr 2021 19:00:15 +0200
Message-ID: <871raumjj4.ffs@nanos.tec.linutronix.de>
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On Wed, Apr 28 2021 at 17:39, Peter Zijlstra wrote:
> On Wed, Apr 28, 2021 at 03:34:52PM +0200, Thomas Gleixner wrote:
>> #4 is the easy case because we can check MSR_TSC_ADJUST to figure out
>>    whether something has written to MSR_TSC or MSR_TSC_ADJUST and undo
>>    the damage in a sane way.
>
> This is after the fact though; userspace (and kernel space) will have
> observed non-linear time and things will be broken in various subtle and
> hard to tell ways.

What I observed in the recent past is that _IF_ that happens it's a
small amount of cycles so it's not a given that this can be observed
accross CPUs. But yes, it's daft.

>> I can live with that and maybe we should have done that 15 years ago
>> instead of trying to work around it at the symptom level.
>
> Anybody that still has runtime BIOS wreckage will then silently suffer
> nonlinear time, doubly so for anybody not having TSC_ADJUST. Are we sure
> we can tell them all to bugger off and buy new hardware?
>
> At the very least we need something like tsc=broken, to explicitly mark
> TSC bad on machines, so that people that see TSC fail on their current
> kernels can continue to use the new kernels. This requires a whole lot
> of care on the part of users though, and will raise a ruckus, because I
> bet a fair number of these people are not even currently aware we're
> disabling TSC for them :/

I'm still allowed to dream, right? :)

Thanks,

        tglx