Re: [PATCH bpf-next v11 08/12] bpf: add BPF_SOCK_OPS_TS_HW_OPT_CB callback

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jason Xing wrote:
> On Sun, Feb 16, 2025 at 6:58 AM Martin KaFai Lau <martin.lau@linux.dev> wrote:
> >
> > On 2/15/25 2:23 PM, Jason Xing wrote:
> > > On Sun, Feb 16, 2025 at 2:08 AM Willem de Bruijn
> > > <willemdebruijn.kernel@gmail.com> wrote:
> > >>
> > >> Jason Xing wrote:
> > >>> On Sat, Feb 15, 2025 at 11:06 PM Willem de Bruijn
> > >>> <willemdebruijn.kernel@gmail.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.
> > >>
> > >> Makes sense.
> > >>
> > >> But that does not ensure that the skb is exclusively owned. Nor that
> > >> the same is true for all drivers using this API (which is not small,
> > >> but small enough to manually review if need be).
> > >>
> > >> The first two examples I happened to look at, i40e and bnx2x, both use
> > >> skb_get() to get a non-exclusive skb reference for their ptp_tx_skb.
> >
> > I think the existing __skb_tstamp_tx() function is also assigning to
> > skb_hwtstamps(skb). The skb may be cloned from the orig_skb first, but they
> > still share the same shinfo. My understanding is that this patch is assigning to
> > the shinfo earlier, so it should not have changed the driver's expectation on
> > the skb_hwtstamps(skb) after calling __skb_tstamp_tx(). If there are drivers
> > assuming exclusive access to the skb_hwtstamps(skb), probably it is something
> > that needs to be addressed regardless and should not be the common case?
> 
> Right, it's also what I was trying to say but missed. Thanks for the
> supplementary info:)

That existing behavior looks dodgy then, too.

I don't have time to look into it deeply right now. But it seems to go
back all the way to the introduction of hw timestamping in commit
ac45f602ee3d in 2009.

I can see how it works in that nothing else holding a clone will
likely have a reason to touch those fields. But that does not make it
correct.

Your point that the new code is no worse than today probably is true.
But when we spot something we prefer to fix it probably. Will need a
deeper look..