linux-kernel - Re: [KVM timekeeping 17/35] Implement getnsboottime kernel API

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Message-ID: <4C77FE57.7050105@siemens.com>
Date:	Fri, 27 Aug 2010 20:05:11 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [KVM timekeeping 17/35] Implement getnsboottime kernel API

Zachary Amsden wrote:
> Add a kernel call to get the number of nanoseconds since boot.  This
> is generally useful enough to make it a generic call.
> 
> Signed-off-by: Zachary Amsden <zamsden@...hat.com>
> ---
>  include/linux/time.h      |    1 +
>  kernel/time/timekeeping.c |   27 +++++++++++++++++++++++++++
>  2 files changed, 28 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/time.h b/include/linux/time.h
> index ea3559f..5d04108 100644
> --- a/include/linux/time.h
> +++ b/include/linux/time.h
> @@ -145,6 +145,7 @@ extern void getnstimeofday(struct timespec *tv);
>  extern void getrawmonotonic(struct timespec *ts);
>  extern void getboottime(struct timespec *ts);
>  extern void monotonic_to_bootbased(struct timespec *ts);
> +extern s64 getnsboottime(void);
>  
>  extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
>  extern int timekeeping_valid_for_hres(void);
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index caf8d4d..d250f0a 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -285,6 +285,33 @@ void ktime_get_ts(struct timespec *ts)
>  }
>  EXPORT_SYMBOL_GPL(ktime_get_ts);
>  
> +
> +/**
> + * getnsboottime - get the bootbased clock in nsec format
> + *
> + * The function calculates the bootbased clock from the realtime
> + * clock and the wall_to_monotonic offset and stores the result
> + * in normalized timespec format in the variable pointed to by @ts.
> + */

This thing is not returning anything in some ts variable. And I also had
a hard time spotting the key difference to getboottime - the name is
really confusing.

Besides this, if you have good suggestion how to provide a compat
version for older kernels, I'm all ears. Please also have a careful look
at kvm-kmod's kvm_getboottime again, right now I'm a bit confused about
what it is supposed to return and what it actually does (note that
kvm-kmod cannot account for time spent in suspend state).

Thanks!
Jan

> +s64 getnsboottime(void)
> +{
> +	unsigned int seq;
> +	s64 secs, nsecs;
> +
> +	WARN_ON(timekeeping_suspended);
> +
> +	do {
> +		seq = read_seqbegin(&xtime_lock);
> +		secs = xtime.tv_sec + wall_to_monotonic.tv_sec;
> +		secs += total_sleep_time.tv_sec;
> +		nsecs = xtime.tv_nsec + wall_to_monotonic.tv_nsec;
> +		nsecs += total_sleep_time.tv_nsec + timekeeping_get_ns();
> +
> +	} while (read_seqretry(&xtime_lock, seq));
> +	return nsecs + (secs * NSEC_PER_SEC);
> +}
> +EXPORT_SYMBOL_GPL(getnsboottime);
> +
>  /**
>   * do_gettimeofday - Returns the time of day in a timeval
>   * @tv:		pointer to the timeval to be set

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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