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

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Message-ID: <4C7BF36F.5080306@siemens.com>
Date:	Mon, 30 Aug 2010 20:07:43 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	"kvm@...r.kernel.org" <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" <linux-kernel@...r.kernel.org>
Subject: Re: [KVM timekeeping 17/35] Implement getnsboottime kernel API

Zachary Amsden wrote:
> On 08/27/2010 08:05 AM, Jan Kiszka wrote:
>> 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
>>>      
>>    
> 
> Yes, we should probably change the name before making this an actual 
> kernel API, John made some better suggestions.
> 
> For kvm-kmod, the following conversion should work:
> 
> static inline u64 get_kernel_ns(void)
> {
>      struct timespec ts;
> 
>      WARN_ON(preemptible());
>      ktime_get_ts(&ts);
>      monotonic_to_bootbased(&ts);
>      return timespec_to_ns(&ts);
> }
> 
> The only real point to getnsboottime is to stop the unnecessary 
> conversions, but looking at it now, it doesn't appear to actually save 
> any, does it?  Maybe it is better to just drop the thing all-together.

It looks like. Will you post a removal patch kvm.git? I'll better wait
with updating kvm-kmod then.

Jan

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