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

[Martin KaFai Lau <martin.lau@linux.dev>]

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".