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

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Date:	Fri, 5 Sep 2014 20:33:22 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Paolo Bonzini <pbonzini@...hat.com>
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

On Fri, 5 Sep 2014, Paolo Bonzini wrote:

> Il 05/09/2014 17:14, Thomas Gleixner ha scritto:
> > So that means the code is correct. Now where is the bug?
> 
> In kernel/time/timekeeping.c?
> 
> We know that we should have
> 
>   base_mono             =     wall_to_monotonic + xtime_sec
> 
> Instead it is
> 
>   base_mono             = wall_to_monotonic + xtime_sec
>                           - seconds from boot time
> 
> which is... zero.  Given this is the only use of base_mono in a
> notifier, I wonder if it is as simple as this (which I don't have time
> to test right now):
> 
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index fb4a9c2cf8d9..f6807a85b8c9 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -443,9 +443,9 @@ static void timekeeping_update(struct timekeeper
> *tk, unsigned int action)
>  		ntp_clear();
>  	}
>  	update_vsyscall(tk);
> -	update_pvclock_gtod(tk, action & TK_CLOCK_WAS_SET);
> 
>  	tk_update_ktime_data(tk);
> +	update_pvclock_gtod(tk, action & TK_CLOCK_WAS_SET);

Why are you moving the update between vsycall and pvclock update as I
did in my patch? We really need to update everything before calling
somewhere.
 
And yes it is that simple. I instrumented the stuff and its correct
now.

Thanks

	tglx
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