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Date:   Wed, 4 Jan 2023 09:46:34 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Feng Tang <feng.tang@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>, x86@kernel.org,
        linux-kernel@vger.kernel.org, rui.zhang@intel.com,
        len.brown@intel.com, tim.c.chen@intel.com
Subject: Re: [PATCH v5] x86/tsc: Add option to force frequency recalibration
 with HW timer
Message-ID: <20230104174634.GA1735127@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20230104081938.1014511-1-feng.tang@intel.com>
 <20230104143227.GC4028633@paulmck-ThinkPad-P17-Gen-1>
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On Wed, Jan 04, 2023 at 06:32:27AM -0800, Paul E. McKenney wrote:
> On Wed, Jan 04, 2023 at 04:19:38PM +0800, Feng Tang wrote:
> > The kernel assumes that the TSC frequency which is provided by the
> > hardware / firmware via MSRs or CPUID(0x15) is correct after applying
> > a few basic consistency checks. This disables the TSC recalibration
> > against HPET or PM timer.
> > 
> > As a result there is no mechanism to validate that frequency in cases
> > where a firmware or hardware defect is suspected. And there was case
> > that some user used atomic clock to measure the TSC frequency and
> > reported an inaccuracy issue, which was later fixed in firmware.
> > 
> > Add an option 'recalibrate' for 'tsc' kernel parameter to force the
> > tsc freq recalibration with HPET or PM timer, and warn if the
> > deviation from previous value is more than about 500 PPM, which
> > provides a way to verify the data from hardware / firmware.
> > 
> > There is no functional change to existing work flow.
> > 
> > Recently there was a real-world case: "The 40ms/s divergence between
> > TSC and HPET was observed on hardware that is quite recent" [1], on
> > that platform the TSC frequence 1896 MHz was got from CPUID(0x15),
> > and the force-reclibration with HPET/PMTIMER both calibrated out
> > value of 1975 MHz, which also matched with check from software
> > 'chronyd', indicating it's a problem of BIOS or firmware.
> > 
> > [Thanks tglx for helping improving the commit log]
> > 
> > [1]. https://lore.kernel.org/lkml/20221117230910.GI4001@paulmck-ThinkPad-P17-Gen-1/
> > Signed-off-by: Feng Tang <feng.tang@intel.com>
> 
> Nice!!!
> 
> Tested-by: Paul E. McKenney <paulmck@kernel.org>

And I have queued this on -rcu for further review and testing, in
particular, to get it into -next sooner rather than later.  Hope that
is OK!

I was thinking that this recalibrate patch made mine unnecessary:

b32498162f5c ("clocksource: Verify HPET and PMTMR when TSC unverified")

But upon further thought, I remembered that what we here at Meta need is
for TSC to remain in use on systems for which it is deemed trustworthy.
The reason is that even a short switch to HPET can terminally annoy some
of our systems.  So I must therefore keep b32498162f5c.

							Thanx, Paul

> > ---
> > Changelog:
> > 
> >   since v4:
> >     * add the real world case, where the patch helped to root
> >       caused a BIOS/FW problem of inaccurate CPUID-0x15 info
> >     * rebase against v6.2-rc1
> > 
> >   since v3:
> >     * add some real world case into commit log
> >     * rebase against v6.0-rc1
> > 
> >   since v2:
> >     * revise the option description in kernel-parameters.txt
> >     * rebase against v5.19-rc2
> > 
> >   since v1:
> >     * refine commit log to state clearly the problem and intention
> >       of the patch by copying Thomas' words.
> > 
> > .../admin-guide/kernel-parameters.txt         |  4 +++
> >  arch/x86/kernel/tsc.c                         | 34 ++++++++++++++++---
> >  2 files changed, 34 insertions(+), 4 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 6cfa6e3996cf..d9eb98e748d5 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -6369,6 +6369,10 @@
> >  			in situations with strict latency requirements (where
> >  			interruptions from clocksource watchdog are not
> >  			acceptable).
> > +			[x86] recalibrate: force to do frequency recalibration
> > +			with a HW timer (HPET or PM timer) for systems whose
> > +			TSC frequency comes from HW or FW through MSR or CPUID(0x15),
> > +			and warn if the difference is more than 500 ppm.
> >  
> >  	tsc_early_khz=  [X86] Skip early TSC calibration and use the given
> >  			value instead. Useful when the early TSC frequency discovery
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index a78e73da4a74..92bbc4a6b3fc 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -48,6 +48,8 @@ static DEFINE_STATIC_KEY_FALSE(__use_tsc);
> >  
> >  int tsc_clocksource_reliable;
> >  
> > +static int __read_mostly tsc_force_recalibrate;
> > +
> >  static u32 art_to_tsc_numerator;
> >  static u32 art_to_tsc_denominator;
> >  static u64 art_to_tsc_offset;
> > @@ -303,6 +305,8 @@ static int __init tsc_setup(char *str)
> >  		mark_tsc_unstable("boot parameter");
> >  	if (!strcmp(str, "nowatchdog"))
> >  		no_tsc_watchdog = 1;
> > +	if (!strcmp(str, "recalibrate"))
> > +		tsc_force_recalibrate = 1;
> >  	return 1;
> >  }
> >  
> > @@ -1374,6 +1378,25 @@ static void tsc_refine_calibration_work(struct work_struct *work)
> >  	else
> >  		freq = calc_pmtimer_ref(delta, ref_start, ref_stop);
> >  
> > +	/* Will hit this only if tsc_force_recalibrate has been set */
> > +	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
> > +
> > +		/* Warn if the deviation exceeds 500 ppm */
> > +		if (abs(tsc_khz - freq) > (tsc_khz >> 11)) {
> > +			pr_warn("Warning: TSC freq calibrated by CPUID/MSR differs from what is calibrated by HW timer, please check with vendor!!\n");
> > +			pr_info("Previous calibrated TSC freq:\t %lu.%03lu MHz\n",
> > +				(unsigned long)tsc_khz / 1000,
> > +				(unsigned long)tsc_khz % 1000);
> > +		}
> > +
> > +		pr_info("TSC freq recalibrated by [%s]:\t %lu.%03lu MHz\n",
> > +			hpet ? "HPET" : "PM_TIMER",
> > +			(unsigned long)freq / 1000,
> > +			(unsigned long)freq % 1000);
> > +
> > +		return;
> > +	}
> > +
> >  	/* Make sure we're within 1% */
> >  	if (abs(tsc_khz - freq) > tsc_khz/100)
> >  		goto out;
> > @@ -1407,8 +1430,10 @@ static int __init init_tsc_clocksource(void)
> >  	if (!boot_cpu_has(X86_FEATURE_TSC) || !tsc_khz)
> >  		return 0;
> >  
> > -	if (tsc_unstable)
> > -		goto unreg;
> > +	if (tsc_unstable) {
> > +		clocksource_unregister(&clocksource_tsc_early);
> > +		return 0;
> > +	}
> >  
> >  	if (boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))
> >  		clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
> > @@ -1421,9 +1446,10 @@ static int __init init_tsc_clocksource(void)
> >  		if (boot_cpu_has(X86_FEATURE_ART))
> >  			art_related_clocksource = &clocksource_tsc;
> >  		clocksource_register_khz(&clocksource_tsc, tsc_khz);
> > -unreg:
> >  		clocksource_unregister(&clocksource_tsc_early);
> > -		return 0;
> > +
> > +		if (!tsc_force_recalibrate)
> > +			return 0;
> >  	}
> >  
> >  	schedule_delayed_work(&tsc_irqwork, 0);
> > -- 
> > 2.34.1
> >