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Date:   Fri, 2 Jun 2023 11:36:54 -0700
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Subject: Re: [PATCH RFC] x86/tsc: Make recalibration default on for
 TSC_KNOWN_FREQ cases
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To:     paulmck@kernel.org, Thomas Gleixner <tglx@linutronix.de>
Cc:     Feng Tang <feng.tang@intel.com>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>, rui.zhang@intel.com,
        x86@kernel.org, linux-kernel@vger.kernel.org
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 <87h6s4ye9b.ffs@tglx> <934e9008-36db-492b-b744-9d68183c6d32@paulmck-laptop>
From:   Dave Hansen <dave.hansen@intel.com>
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On 6/2/23 11:29, Paul E. McKenney wrote:
>>> One downside is, many VMs also has X86_FEATURE_TSC_KNOWN_FREQ set,
>>> and they will also do this recalibration.
>> It's also pointless for those SoCs which lack legacy hardware.
>>
>> So why do you force this on everyone?
> Just for the record, this patch could be helpful in allowing victims
> of TSC mis-synchronization to more easily provide a more complete bug
> report to the firmware people.  There is of course no point if there is
> already a fix available.
> 
> But it is not all that hard to work around not having this patch upstream.
> This can be hand-applied as needed, NTP drift rates can be pressed
> into service for those of us having atomic clocks near all our servers,
> or the firmware guys can be tasked with figuring it out.
> 
> So this patch would be nice to have, but we could live without it.

Is this the kind of thing we could relegate to a kernel unit test?  Like
make the recalibration logic _available_, but don't have it affect the
rest of the system.

I love patching my kernel as much as the next guy.  But, you know what I
*don't* love?  Explaining how to patch kernels to other people. ;)