netdev - Re: [PATCH net-next v4 07/11] net-timestamp: support hwtstamp print for bpf extension

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Message-ID: <53c3be2f-1d5d-44cb-8c27-18c84bc30c9e@linux.dev>
Date: Fri, 13 Dec 2024 15:15:48 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, dsahern@...nel.org, willemdebruijn.kernel@...il.com,
 willemb@...gle.com, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org,
 netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v4 07/11] net-timestamp: support hwtstamp print
 for bpf extension

On 12/13/24 7:13 AM, Jason Xing wrote:
>>> -static void __skb_tstamp_tx_bpf(struct sock *sk, struct sk_buff *skb, int tstype)
>>> +static void __skb_tstamp_tx_bpf(struct sock *sk, struct sk_buff *skb,
>>> +                             struct skb_shared_hwtstamps *hwtstamps,
>>> +                             int tstype)
>>>    {
>>> +     struct timespec64 tstamp;
>>> +     u32 args[2] = {0, 0};
>>>        int op;
>>>
>>>        if (!sk)
>>> @@ -5552,6 +5556,11 @@ static void __skb_tstamp_tx_bpf(struct sock *sk, struct sk_buff *skb, int tstype
>>>                break;
>>>        case SCM_TSTAMP_SND:
>>>                op = BPF_SOCK_OPS_TS_SW_OPT_CB;
>>> +             if (hwtstamps) {
>>> +                     tstamp = ktime_to_timespec64(hwtstamps->hwtstamp);
>> Avoid this conversion which is likely not useful to the bpf prog. Directly pass
>> hwtstamps->hwtstamp (in ns?) to the bpf prog. Put lower 32bits in args[0] and
>> higher 32bits in args[1].
> It makes sense.

When replying the patch 2 thread, I noticed it may not even have to pass the 
hwtstamps in args here.

Can "*skb_hwtstamps(skb) = *hwtstamps;" be done before calling the bpf prog? 
Then the bpf prog can directly get it from skb_shinfo(skb)->hwtstamps.
It is like reading other fields in skb_shinfo(skb), e.g. the 
skb_shinfo(skb)->tskey discussed in patch 10. The bpf prog will have a more 
consistent experience in reading different fields of the skb_shinfo(skb). 
skb_shinfo(skb)->hwtstamps is a more intuitive place to obtain the hwtstamp than 
the broken up args[0] and args[1]. On top of that, there is also an older 
"skb_hwtstamp" field in "struct bpf_sock_ops".