linux-kernel - Re: [PATCH v3 2/2] hv_utils: implement Hyper-V PTP source

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Message-ID: <20170117083547.3c7d71a4@xeon-e3>
Date:   Tue, 17 Jan 2017 08:35:47 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     devel@...uxdriverproject.org, Thomas Gleixner <tglx@...utronix.de>,
        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>, Olaf Hering <olaf@...fle.de>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH v3 2/2] hv_utils: implement Hyper-V PTP source

On Tue, 17 Jan 2017 16:27:19 +0100
Vitaly Kuznetsov <vkuznets@...hat.com> 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 may 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.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Looks good. Minor style comments.

> ---
>  drivers/hv/hv_util.c | 140 ++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 115 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/hv/hv_util.c b/drivers/hv/hv_util.c
> index 94719eb..e49c5f3 100644
> --- a/drivers/hv/hv_util.c
> +++ b/drivers/hv/hv_util.c

 
> +static inline u64 get_timeadj_latency(u64 ref_time)

inline not necessary on static functions. GCC inlines anyway

> +{
> +	u64 current_tick;
> +
> +	if (ts_srv_version <= TS_VERSION_3)
> +		return 0;
> +
> +	/*
> +	 * Some latency has been introduced since Hyper-V generated
> +	 * its time sample. Take that latency into account before
> +	 * using TSC reference time sample from Hyper-V.
> +	 *
> +	 * This sample is given by TimeSync v4 and above hosts.
> +	 */
> +
> +	rdmsrl(HV_X64_MSR_TIME_REF_COUNT, current_tick);

Personal preference is not to add blank line between comment
and associated code.

...

> +
> +struct ptp_clock_info ptp_hyperv_info = {

This could be static?
Could it be const?

> +	.name		= "hyperv",
> +	.enable         = hv_ptp_enable,
> +	.adjtime        = hv_ptp_adjtime,
> +	.adjfreq        = hv_ptp_adjfreq,
> +	.gettime64      = hv_ptp_gettime,
> +	.settime64      = hv_ptp_settime,
> +	.owner		= THIS_MODULE,
> +};
> +
> +static struct ptp_clock *hv_ptp_clock;
> +
>  static int hv_timesync_init(struct hv_util_service *srv)
>  {
>  	INIT_WORK(&wrk.work, hv_set_host_time);
> +
> +	hv_ptp_clock = ptp_clock_register(&ptp_hyperv_info, NULL);
> +	if (IS_ERR_OR_NULL(hv_ptp_clock)) {
> +		pr_err("cannot register PTP clock: %ld\n",
> +		       PTR_ERR(hv_ptp_clock));

Why not return error to init routine in case of failure.

> +		hv_ptp_clock = NULL;

Why not return error to init routine?  Rather than having user
scan log.

> +	}
> +
>  	return 0;
>  }