linux-kernel - Re: [PATCH RFC] x86/tsc: Make recalibration default on for TSC_KNOWN

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Message-ID: <73283eba-14b8-2da1-6109-c183586fe5b7@intel.com>
Date:   Fri, 2 Jun 2023 11:36:54 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     paulmck@...nel.org, Thomas Gleixner <tglx@...utronix.de>
Cc:     Feng Tang <feng.tang@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, rui.zhang@...el.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] x86/tsc: Make recalibration default on for
 TSC_KNOWN_FREQ cases

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. ;)