From: Thomas Gleixner <tglx@linutronix.de>
To: Feng Tang <feng.tang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
John Stultz <john.stultz@linaro.org>,
Stephen Boyd <sboyd@kernel.org>,
linux-kernel@vger.kernel.org, Qais Yousef <qais.yousef@arm.com>,
andi.kleen@intel.com
Subject: Re: [PATCH] clocksource: don't run watchdog forever
Date: Thu, 04 Mar 2021 15:15:13 +0100 [thread overview]
Message-ID: <87wnun57ke.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20210304074316.GA43191@shbuild999.sh.intel.com>
Feng,
On Thu, Mar 04 2021 at 15:43, Feng Tang wrote:
> On Wed, Mar 03, 2021 at 04:50:31PM +0100, Thomas Gleixner wrote:
>> Anything pre TSC_ADJUST wants the watchdog on. With TSC ADJUST available
>> we can probably avoid it.
>>
>> There is a caveat though. If the machine never goes idle then TSC adjust
>> is not able to detect a potential wreckage. OTOH, most of the broken
>> BIOSes tweak TSC only by a few cycles and that is usually detectable
>> during boot. So we might be clever about it and schedule a check every
>> hour when during the first 10 minutes a modification of TSC adjust is
>> seen on any CPU.
>
> I don't have much experience with tsc_adjust, and try to understand it:
> The 'modification of TSC' here has 2 cases: ?
> * First read of 'TSC_ADJUST' MSR of a just boot CPU returns non-zero
> value
That's catching stupid BIOSes which set the TSC to random values during
boot/reboot. That's a one off boot issue and not a real problem. The
kernel fixes it up and is done with it. Nothing to care about.
> * Following read of 'TSC_ADJUST' doesn't equal to the 'tsc_adjust' value
> saved in per-cpu data.
That's where we catch broken BIOS/SMI implementations which try to
"hide" the time wasted in BIOS/SMI by setting the TSC back to the value
they saved on SMI entry. That was a popular BIOS "feature" some years
ago, but it seems the BIOS tinkerers finally gave up on it.
>> Where is this TSC_DISABLE_WRITE bit again?
I'm serious about this. Once the kernel has taken over a CPU there is
absolutely no reason for any context to write to the TSC/TSC_ADJUST
register ever again. So having a mechanism to prevent writes would
surely help to make the TSC more trustworthy.
> Also, does the patch ("x86/tsc: mark tsc reliable for qualified platforms")
> need to wait till this caveat case is solved?
Yes, but that should be trivial to do.
Thanks,
tglx
next prev parent reply other threads:[~2021-03-04 14:17 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-02 2:54 [PATCH] clocksource: don't run watchdog forever Feng Tang
2021-03-02 9:16 ` Peter Zijlstra
2021-03-02 12:06 ` Feng Tang
2021-03-03 15:50 ` Thomas Gleixner
2021-03-04 7:43 ` Feng Tang
2021-03-04 14:15 ` Thomas Gleixner [this message]
2021-03-05 2:30 ` Feng Tang
2021-03-25 8:34 ` Feng Tang
2021-03-25 11:40 ` Thomas Gleixner
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