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Date:   Thu, 30 Nov 2023 11:16:43 -0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     pmartincic@linux.microsoft.com
Cc:     "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] hv_utils: Allow implicit ICTIMESYNCFLAG_SYNC
Message-ID: <ZWjfm25xAqvbQYSf@boqun-archlinux>
References: <20231127213524.52783-1-pmartincic@linux.microsoft.com>
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In-Reply-To: <20231127213524.52783-1-pmartincic@linux.microsoft.com>
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On Mon, Nov 27, 2023 at 01:35:24PM -0800, pmartincic@linux.microsoft.com wrote:
> From: Peter Martincic <pmartincic@microsoft.com>
> 
> Hyper-V hosts can omit the _SYNC flag to due a bug on resume from modern
> suspend. In such a case, the guest may fail to update its time-of-day to
> account for the period when it was suspended, and could proceed with a
> significantly wrong time-of-day. In such a case when the guest is
> significantly behind, fix it by treating a _SAMPLE the same as if _SYNC
> was received so that the guest time-of-day is updated.
> 
> This is hidden behind param hv_utils.timesync_implicit.
> 
> Signed-off-by: Peter Martincic <pmartincic@microsoft.com>

Looks good to me.

Acked-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> ---
>  drivers/hv/hv_util.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/hv_util.c b/drivers/hv/hv_util.c
> index 42aec2c5606a..9c97c4065fe7 100644
> --- a/drivers/hv/hv_util.c
> +++ b/drivers/hv/hv_util.c
> @@ -296,6 +296,11 @@ static struct {
>  	spinlock_t			lock;
>  } host_ts;
>  
> +static bool timesync_implicit;
> +
> +module_param(timesync_implicit, bool, 0644);
> +MODULE_PARM_DESC(timesync_implicit, "If set treat SAMPLE as SYNC when clock is behind");
> +
>  static inline u64 reftime_to_ns(u64 reftime)
>  {
>  	return (reftime - WLTIMEDELTA) * 100;
> @@ -344,6 +349,29 @@ static void hv_set_host_time(struct work_struct *work)
>  		do_settimeofday64(&ts);
>  }
>  
> +/*
> + * Due to a bug on Hyper-V hosts, the sync flag may not always be sent on resume.
> + * Force a sync if the guest is behind.
> + */
> +static inline bool hv_implicit_sync(u64 host_time)
> +{
> +	struct timespec64 new_ts;
> +	struct timespec64 threshold_ts;
> +
> +	new_ts = ns_to_timespec64(reftime_to_ns(host_time));
> +	ktime_get_real_ts64(&threshold_ts);
> +
> +	threshold_ts.tv_sec += 5;
> +
> +	/*
> +	 * If guest behind the host by 5 or more seconds.
> +	 */
> +	if (timespec64_compare(&new_ts, &threshold_ts) >= 0)
> +		return true;
> +
> +	return false;
> +}
> +
>  /*
>   * Synchronize time with host after reboot, restore, etc.
>   *
> @@ -384,7 +412,8 @@ static inline void adj_guesttime(u64 hosttime, u64 reftime, u8 adj_flags)
>  	spin_unlock_irqrestore(&host_ts.lock, flags);
>  
>  	/* Schedule work to do do_settimeofday64() */
> -	if (adj_flags & ICTIMESYNCFLAG_SYNC)
> +	if ((adj_flags & ICTIMESYNCFLAG_SYNC) ||
> +	    (timesync_implicit && hv_implicit_sync(host_ts.host_time)))
>  		schedule_work(&adj_time_work);
>  }
>  
> -- 
> 2.34.1
>