On Tue, 17 Jan 2017 16:27:19 +0100
Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> 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@xxxxxxxxxx>
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;
> }
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