linux-kernel - Re: [PATCH] x86: tsc: fix L2 guest always in tsc

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Message-ID: <20180423193723.GO4082@hirez.programming.kicks-ass.net>
Date:   Mon, 23 Apr 2018 21:37:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peng Hao <peng.hao2@....com.cn>
Cc:     tglx@...utronix.de, pbonzini@...hat.com, mingo@...hat.com,
        x86@...nel.org, len.brown@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: tsc: fix L2 guest always in tsc_early clocksource

On Tue, Apr 24, 2018 at 09:08:54AM +0800, Peng Hao wrote:
> In L2 guest tsc_read_refs always return ULLONG_MAX, and that will
> call tsc_refine_calibration_work periodly. So L2 guest will read
> acpi timer port 0x608 periodly.
> The patch will let it out of "if(tsc_start == -1){}" infinite loop.

Help me out a little. What's an L2 guest?

And I'm assuming you don't have HPET and then end up wanting to use
PM-timer? Which is also busted and resturns crap results?

So if the whole TSC sync has no way of ever working; what is your patch
doing? How is the actual TSC calibrated?

Should you instead be using fast_calibrate() and passing in a known
value from the host? Thereby avoiding all this calibration nonsense
entirely.