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

[Jason Xing <kerneljasonxing@gmail.com>]

On Tue, Feb 18, 2025 at 8:56 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> Jason Xing wrote:
> > On Sun, Feb 16, 2025 at 10:45 PM Jason Xing <kerneljasonxing@gmail.com> wrote:
> > >
> > > On Sun, Feb 16, 2025 at 10:36 PM Willem de Bruijn
> > > <willemdebruijn.kernel@gmail.com> wrote:
> > > >
> > > > 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.
> > >
> > > Right. And hardware timestamping has been used for many years, I presume.
> > >
> > > >
> > > > 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.
> > >
> > > Right.
> > >
> > > > But when we spot something we prefer to fix it probably. Will need a
> > > > deeper look..
> > >
> > > Got it. I added it to my to-do list. If you don't mind, I plan to take
> > > a deep look in March and then get back to you because recently I'm
> > > occupied by many things. I need to study some of the drivers that
> > > don't use skb_get() there.
> > >
> >
> > Oh, sorry, I forgot to ask: what should we do next regarding this series ?
>
> Please resubmit with the two remaining small issues addressed:
>
> - Martin's point about moving code to patch 8
> - Remove unused variable

Sure, the v12 is coming soon :)

Thanks,
Jason