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Date:   Tue, 17 Jan 2017 10:53:46 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        John Stultz <john.stultz@...aro.org>,
        Alex Ng <alexng@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH RFC] hv_utils: implement Hyper-V PTP source

Thomas Gleixner <tglx@...utronix.de> writes:

> On Fri, 13 Jan 2017, Vitaly Kuznetsov wrote:
>> With TimeSync version 4 protocol support we started updating system time
>> continuously through the whole lifetime of Hyper-V guests. Every 5 seconds
>> there is a time sample from the host which triggers do_settimeofday[64]().
>> While the time from the host is very accurate such adjustments may cause
>> issues:
>> - Time is jumping forward and backward, some applications may misbehave.
>> - In case an NTP server runs in parallel and uses something else for time
>>   sync (network, PTP,...) system time will never converge.
>> - Systemd starts annoying you by printing "Time has been changed" every 5
>>   seconds to the system log.
>> 
>> Instead of doing in-kernel time adjustments offload the work to an
>> NTP client by exposing TimeSync messages as a PTP device. Users my now
>> decide what they want to use as a source.
>> 
>> I tested the solution with chrony, the config was:
>> 
>>  refclock PHC /dev/ptp0 poll 3 precision 1e-9
>> 
>> The result I'm seeing is accurate enough, the time delta between the guest
>> and the host is almost always within [-10us, +10us], the in-kernel solution
>> was giving us comparable results.
>> 
>> I also tried implementing PPS device instead of PTP by using not currently
>> used Hyper-V synthetic timers (we use only one of four for clockevent) but
>> with PPS source only chrony wasn't able to give me the required accuracy,
>> the delta often more that 100us.
>
> Makes sense. The PTP based solution is really nice!
>
>>  static void hv_set_host_time(struct work_struct *work)
>>  {
>>  	struct adj_time_work	*wrk;
>> -	s64 host_tns;
>>  	u64 newtime;
>>  	struct timespec64 host_ts;
>
> Just a nitpick. Ordering variables in reverse fir tree (length) order:
>
>   	struct adj_time_work *wrk;
>   	struct timespec64 host_ts;
>   	u64 newtime;
>
> makes is simpler to parse
>
>> +
>> +static struct {
>> +	u64 host_time;
>> +	u64 ref_time;
>> +	spinlock_t lock;
>> +} host_ts;
>
> Another formatting nit. If you arrange the members in tabular fashion it
> becomes simpler to parse:
>
> static struct {
> 	u64		host_time;
> 	u64		ref_time;
> 	spinlock_t 	lock;
> } host_ts;
>
> Also the struct might do with some comment explaning that it is the storage
> for the PTP machinery,
>
>> +static inline void adj_guesttime(u64 hosttime, u64 reftime, u8 adj_flags)
>>  {
>> +	unsigned long flags;
>>  
>>  	/*
>>  	 * This check is safe since we are executing in the
>>  	 * interrupt context and time synch messages arre always
>>  	 * delivered on the same CPU.
>>  	 */
>> -	if (work_pending(&wrk.work))
>> -		return;
>> +	if (adj_flags & ICTIMESYNCFLAG_SYNC) {
>> +		if (work_pending(&wrk.work))
>> +			return;
>>  
>> -	wrk.host_time = hosttime;
>> -	wrk.ref_time = reftime;
>> -	wrk.flags = flags;
>> -	if ((flags & (ICTIMESYNCFLAG_SYNC | ICTIMESYNCFLAG_SAMPLE)) != 0) {
>> +		wrk.host_time = hosttime;
>> +		wrk.ref_time = reftime;
>> +		wrk.flags = adj_flags;
>>  		schedule_work(&wrk.work);
>> +	} else {
>> +		spin_lock_irqsave(&host_ts.lock, flags);
>> +		host_ts.host_time = hosttime;
>> +
>> +		if (ts_srv_version <= TS_VERSION_3)
>> +			rdmsrl(HV_X64_MSR_TIME_REF_COUNT, host_ts.ref_time);
>
> I'm confused here. The reftime / hosttime pair is accurate at sampling time
> on the host. So why reading the MSR here? I'm certainly missing something,
> but then this wants to have a comment like the other one in
> get_timeadj_latency().

Old TimeSync (vesion < 4.0) protocol messages don't specify any
reference time so we're 'faking' it by saving the reference time when
the message was received on the guest. This, of course, reduces the
precision of the device as we have a delta between the time sample
generation and its reception on the guest but there is no way we can
calculate this delta. I'll put a comment.

>
>> +		else
>> +			host_ts.ref_time = reftime;
>> +		spin_unlock_irqrestore(&host_ts.lock, flags);
>>  	}
>>  }
>
> Other than that: Nice work!
>

Thanks, I'll incorporate all your feedback and post non-RFC version.

-- 
  Vitaly

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