linux-kernel - Re: [PATCH] KVM: x86: fix kvmclock breakage from timers branch merge

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Date:	Thu, 04 Sep 2014 23:22:29 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org, chris.j.arges@...onical.com,
	kvm@...r.kernel.org, John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH] KVM: x86: fix kvmclock breakage from timers branch merge

Il 04/09/2014 22:58, Thomas Gleixner ha scritto:
> This is simply wrong.

It is.

> Now I have no idea why you think it needs to add xtime_sec. If the
> result is wrong, then we need to figure out which one of the supplied
> values is wrong and not blindly add xtime_sec just because that makes
> it magically correct.
> 
> Can you please provide a proper background why you think that adding
> xtime_sec is a good idea?

It's not a good idea indeed.  I didn't fully digest the 3.16->3.17
timekeeping changes and messed up this patch.

However, there is a bug in the "base_mono + offs_boot" formula, given
that:

- bisection leads to the merge commit of John's timers branch

- bisecting within John's timers branch, with a KVM commit on top to
  make the code much easier to trigger, leads to commit cbcf2dd3b3d4
  (x86: kvm: Make kvm_get_time_and_clockread() nanoseconds based,
  2014-07-16).

- I backported your patch to 3.16, using wall_to_monotonic +
  total_sleep_time + xtime_sec (wtm+xtime_sec as in pre-cbcf2dd3b3d4
  code, total_sleep_time from 3.16 monotonic_to_bootbased) and it works

- In v2 of the patch I fixed the bug by changing the formula
  "base_mono + offs_boot" to "offs_boot - offs_real" (and then adding
  xtime_sec separately as in the 3.16 backport), but the two formulas
  "base_mono + offs_boot" and "offs_boot - offs_real + xtime_sec" ought
  to be identical.

I find "offs_boot - offs_real + xtime" more readable than the
alternative "base_mono + offs_boot + xtime_nsec", so the fix doubles as
a cleanup for me and I'm fine with it.  But something must be wrong in
the timekeeping code.

Paolo
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