Re: [PATCH net-next v6 1/2] net-timestamp: introduce SOF_TIMESTAMPING_OPT_RX_FILTER flag

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

Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> introduce a new flag SOF_TIMESTAMPING_OPT_RX_FILTER in the receive
> path. User can set it with SOF_TIMESTAMPING_SOFTWARE to filter
> out rx software timestamp report, especially after a process turns on
> netstamp_needed_key which can time stamp every incoming skb.
> 
> Previously, we found out if an application starts first which turns on
> netstamp_needed_key, then another one only passing SOF_TIMESTAMPING_SOFTWARE
> could also get rx timestamp. Now we handle this case by introducing this
> new flag without breaking users.
> 
> Quoting Willem to explain why we need the flag:
> "why a process would want to request software timestamp reporting, but
> not receive software timestamp generation. The only use I see is when
> the application does request
> SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_TX_SOFTWARE."
> 
> Similarly, this new flag could also be used for hardware case where we
> can set it with SOF_TIMESTAMPING_RAW_HARDWARE, then we won't receive
> hardware receive timestamp.
> 
> Another thing about errqueue in this patch I have a few words to say:
> In this case, we need to handle the egress path carefully, or else
> reporting the tx timestamp will fail. Egress path and ingress path will
> finally call sock_recv_timestamp(). We have to distinguish them.
> Errqueue is a good indicator to reflect the flow direction.
> 
> Suggested-by: Willem de Bruijn <willemb@google.com>
> Signed-off-by: Jason Xing <kernelxing@tencent.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

I really only suggested making this a new flag, not the main idea of
filtering.

> ---
> v6
> Link: https://lore.kernel.org/all/20240906095640.77533-1-kerneljasonxing@gmail.com/
> 1. add the description in doc provided by Willem
> 2. align the if statements (Willem)
> 
> v5
> Link: https://lore.kernel.org/all/20240905071738.3725-1-kerneljasonxing@gmail.com/
> 1. squash the hardware case patch into this one (Willem)
> 2. update corresponding commit message and doc (Willem)
> 3. remove the limitation in sock_set_timestamping() and restore the
> simplification branches. (Willem)
> 
> v4
> Link: https://lore.kernel.org/all/20240830153751.86895-2-kerneljasonxing@gmail.com/
> 1. revise the commit message and doc (Willem)
> 2. simplify the test statement (Jakub)
> 3. add Willem's reviewed-by tag (Willem)
> 
> v3
> 1. Willem suggested this alternative way to solve the issue, so I
> added his Suggested-by tag here. Thanks!
> ---
>  Documentation/networking/timestamping.rst | 17 +++++++++++++++++
>  include/uapi/linux/net_tstamp.h           |  3 ++-
>  net/ethtool/common.c                      |  1 +
>  net/ipv4/tcp.c                            |  9 +++++++--
>  net/socket.c                              | 10 ++++++++--
>  5 files changed, 35 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> index 9c7773271393..8199e6917671 100644
> --- a/Documentation/networking/timestamping.rst
> +++ b/Documentation/networking/timestamping.rst
> @@ -267,6 +267,23 @@ SOF_TIMESTAMPING_OPT_TX_SWHW:
>    two separate messages will be looped to the socket's error queue,
>    each containing just one timestamp.
>  
> +SOF_TIMESTAMPING_OPT_RX_FILTER:
> +  Filter out spurious receive timestamps: report a receive timestamp
> +  only if the matching timestamp generation flag is enabled.
> +
> +  Receive timestamps are generated early in the ingress path, before a
> +  packet's destination socket is known. If any socket enables receive
> +  timestamps, packets for all socket will receive timestamped packets.

nit: s/packets for all socket/all sockets/

My error in my suggestion.

Not important enough to respin.

> +  Including those that request timestamp reporting with
> +  SOF_TIMESTAMPING_SOFTWARE and/or SOF_TIMESTAMPING_RAW_HARDWARE, but
> +  do not request receive timestamp generation. This can happen when
> +  requesting transmit timestamps only.