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

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Date:	Thu, 04 Sep 2014 19:56:35 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org
CC:	chris.j.arges@...onical.com, kvm@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH] KVM: x86: fix kvmclock breakage from timers branch merge

Il 04/09/2014 14:58, Paolo Bonzini ha scritto:
> Commit cbcf2dd3b3d4 (x86: kvm: Make kvm_get_time_and_clockread() nanoseconds
> based, 2014-07-16) forgot to add tk->xtime_sec, thus breaking kvmclock on
> hosts that have a reliable TSC.  Add it back; and since the field boot_ns
> is not anymore related to the host boot-based clock, rename boot_ns->nsec_base
> and the existing nsec_base->snsec_base.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: John Stultz <john.stultz@...aro.org>
> Reported-by: Chris J Arges <chris.j.arges@...onical.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8f1e22d3b286..92493e10937c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1020,8 +1020,8 @@ struct pvclock_gtod_data {
>  		u32	shift;
>  	} clock;
>  
> -	u64		boot_ns;
>  	u64		nsec_base;
> +	u64		snsec_base;
>  };
>  
>  static struct pvclock_gtod_data pvclock_gtod_data;
> @@ -1042,8 +1042,9 @@ static void update_pvclock_gtod(struct timekeeper *tk)
>  	vdata->clock.mult		= tk->tkr.mult;
>  	vdata->clock.shift		= tk->tkr.shift;
>  
> -	vdata->boot_ns			= boot_ns;
> -	vdata->nsec_base		= tk->tkr.xtime_nsec;
> +	vdata->nsec_base		= tk->xtime_sec * (u64)NSEC_PER_SEC
> +					+ boot_ns;
> +	vdata->snsec_base		= tk->tkr.xtime_nsec;

Hmm, I found this comment in kernel/time/timekeeping.c

        /*
         * The xtime based monotonic readout is:
         *      nsec = (xtime_sec + wtm_sec) * 1e9 + wtm_nsec + now();
         * The ktime based monotonic readout is:
         *      nsec = base_mono + now();

so this patch makes no sense.  The offs_boot part must be broken.

Paolo

>  
>  	write_seqcount_end(&vdata->seq);
>  }
> @@ -1413,10 +1414,10 @@ static int do_monotonic_boot(s64 *t, cycle_t *cycle_now)
>  	do {
>  		seq = read_seqcount_begin(&gtod->seq);
>  		mode = gtod->clock.vclock_mode;
> -		ns = gtod->nsec_base;
> +		ns = gtod->snsec_base;
>  		ns += vgettsc(cycle_now);
>  		ns >>= gtod->clock.shift;
> -		ns += gtod->boot_ns;
> +		ns += gtod->nsec_base;
>  	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
>  	*t = ns;
>  
> 

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