netdev - Re: [PATCH bpf-next v11 08/12] bpf: add BPF_SOCK_OPS_TS_HW_OPT

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Message-ID: <CAL+tcoAJHSfBrfdn-Cmk=9ZkMNSdkGYKJbZ0mynn_=qU9Mp1Ag@mail.gmail.com>
Date: Sun, 16 Feb 2025 00:20:29 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, willemb@...gle.com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	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, horms@...nel.org, bpf@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v11 08/12] bpf: add BPF_SOCK_OPS_TS_HW_OPT_CB callback

On Sat, Feb 15, 2025 at 11:06 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > Support hw SCM_TSTAMP_SND case for bpf timestamping.
> >
> > Add a new sock_ops callback, BPF_SOCK_OPS_TS_HW_OPT_CB. This
> > callback will occur at the same timestamping point as the user
> > space's hardware SCM_TSTAMP_SND. The BPF program can use it to
> > get the same SCM_TSTAMP_SND timestamp without modifying the
> > user-space application.
> >
> > To avoid increasing the code complexity, replace SKBTX_HW_TSTAMP
> > with SKBTX_HW_TSTAMP_NOBPF instead of changing numerous callers
> > from driver side using SKBTX_HW_TSTAMP. The new definition of
> > SKBTX_HW_TSTAMP means the combination tests of socket timestamping
> > and bpf timestamping. After this patch, drivers can work under the
> > bpf timestamping.
> >
> > Considering some drivers doesn't assign the skb with hardware
> > timestamp,
>
> This is not for a real technical limitation, like the skb perhaps
> being cloned or shared?

Agreed on this point. I'm kind of familiar with I40E, so I dare to say
the reason why it doesn't assign the hwtstamp is because the skb will
soon be destroyed, that is to say, it's pointless to assign the
timestamp.

>
> > this patch do the assignment and then BPF program
> > can acquire the hwstamp from skb directly.
>
> If the above is not the case and it is safe to write to the skb_shinfo,
> and only if respinning anyway, grammar:

>From what I've known about various drivers (although very limited),
it's safe to do the assignment.

>
> s/doesn't/don't/
> s/do/does/

Thanks for catching these things. If the re-spin is necessary, I will
fix them all for sure.

Thanks,
Jason