Re: [PATCH net-next 1/6] bpf: Add bpf_ktime_get_real_ns() kfunc

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 18/05/2026 10.23, Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> Currently BPF programs can obtain timestamps via bpf_ktime_get_ns(),
> which returns CLOCK_MONOTONIC time. However, the skb->tstamp field
> populated by the network stack uses ktime_get_real() which is in the
> CLOCK_REALTIME domain.
> 
> In the series, kernel reports the software/hardware timestamps through
> sockopt and then userspace bpf application gets them and calculate the
> delta only through an unified time unit.
> 
> However, prior to this, when a BPF program tries to measure RX packet
> delay by comparing skb->tstamp with bpf_ktime_get_ns(), the result
> is incorrect because the two clocks have different epochs.
> 
> Introduce a new BPF kfunc bpf_ktime_get_real_ns() that returns the
> current CLOCK_REALTIME time. This allows BPF programs to perform
> accurate delay calculations without clock domain mismatch issue.
> 

I support adding this helper, because have also hit this issue[1].
Our ugly workaround is to use the TAI clock and adjust for the current 
TAI offset.

[1] https://github.com/xdp-project/bpf-examples/tree/main/netstacklat

Cc Simon Sundberg <Simon.Sundberg@kau.se>

> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
>   kernel/bpf/helpers.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 2bb60200c266..863645d096ef 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2317,6 +2317,11 @@ void bpf_rb_root_free(const struct btf_field *field, void *rb_root,
>   
>   __bpf_kfunc_start_defs();
>   
> +__bpf_kfunc u64 bpf_ktime_get_real_ns(void)
> +{
> +	return ktime_get_real_fast_ns();
> +}
> +
>   /**
>    * bpf_obj_new() - allocate an object described by program BTF
>    * @local_type_id__k: type ID in program BTF
> @@ -4859,6 +4864,7 @@ BTF_ID_FLAGS(func, bpf_task_work_schedule_resume, KF_IMPLICIT_ARGS)
>   BTF_ID_FLAGS(func, bpf_dynptr_from_file)
>   BTF_ID_FLAGS(func, bpf_dynptr_file_discard)
>   BTF_ID_FLAGS(func, bpf_timer_cancel_async)
> +BTF_ID_FLAGS(func, bpf_ktime_get_real_ns)
>   BTF_KFUNCS_END(common_btf_ids)
>   
>   static const struct btf_kfunc_id_set common_kfunc_set = {