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

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 4 Sep 2014, Paolo Bonzini wrote:
> 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.

I think I have a vague idea what happened, but I'm way too tired now
to write it up fully. I'll do that tomorrow morning with brain awake.

Thanks,

	tglx