Re: [PATCH v2 5/5] io_uring/netcmd: add tx timestamping cmd support

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

Vadim Fedorenko wrote:
> On 05/06/2025 01:59, Willem de Bruijn wrote:
> > Pavel Begunkov wrote:
> >> Add a new socket command which returns tx time stamps to the user. It
> >> provide an alternative to the existing error queue recvmsg interface.
> >> The command works in a polled multishot mode, which means io_uring will
> >> poll the socket and keep posting timestamps until the request is
> >> cancelled or fails in any other way (e.g. with no space in the CQ). It
> >> reuses the net infra and grabs timestamps from the socket's error queue.
> >>
> >> The command requires IORING_SETUP_CQE32. All non-final CQEs (marked with
> >> IORING_CQE_F_MORE) have cqe->res set to the tskey, and the upper 16 bits
> >> of cqe->flags keep tstype (i.e. offset by IORING_CQE_BUFFER_SHIFT). The
> >> timevalue is store in the upper part of the extended CQE. The final
> >> completion won't have IORING_CQR_F_MORE and will have cqe->res storing
> >> 0/error.
> >>
> >> Suggested-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
> >> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> >> ---
> >>   include/uapi/linux/io_uring.h |  6 +++
> >>   io_uring/cmd_net.c            | 77 +++++++++++++++++++++++++++++++++++
> >>   2 files changed, 83 insertions(+)
> >>
> >> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> >> index cfd17e382082..0bc156eb96d4 100644
> >> --- a/include/uapi/linux/io_uring.h
> >> +++ b/include/uapi/linux/io_uring.h
> >> @@ -960,6 +960,11 @@ struct io_uring_recvmsg_out {
> >>   	__u32 flags;
> >>   };
> >>   
> >> +struct io_timespec {
> >> +	__u64		tv_sec;
> >> +	__u64		tv_nsec;
> >> +};
> >> +
> >>   /*
> >>    * Argument for IORING_OP_URING_CMD when file is a socket
> >>    */
> >> @@ -968,6 +973,7 @@ enum io_uring_socket_op {
> >>   	SOCKET_URING_OP_SIOCOUTQ,
> >>   	SOCKET_URING_OP_GETSOCKOPT,
> >>   	SOCKET_URING_OP_SETSOCKOPT,
> >> +	SOCKET_URING_OP_TX_TIMESTAMP,
> >>   };
> >>   
> >>   /* Zero copy receive refill queue entry */
> >> diff --git a/io_uring/cmd_net.c b/io_uring/cmd_net.c
> >> index e99170c7d41a..dae59aea5847 100644
> >> --- a/io_uring/cmd_net.c
> >> +++ b/io_uring/cmd_net.c
> >> @@ -1,5 +1,6 @@
> >>   #include <asm/ioctls.h>
> >>   #include <linux/io_uring/net.h>
> >> +#include <linux/errqueue.h>
> >>   #include <net/sock.h>
> >>   
> >>   #include "uring_cmd.h"
> >> @@ -51,6 +52,80 @@ static inline int io_uring_cmd_setsockopt(struct socket *sock,
> >>   				  optlen);
> >>   }
> >>   
> >> +static bool io_process_timestamp_skb(struct io_uring_cmd *cmd, struct sock *sk,
> >> +				     struct sk_buff *skb, unsigned issue_flags)
> >> +{
> >> +	struct sock_exterr_skb *serr = SKB_EXT_ERR(skb);
> >> +	struct io_uring_cqe cqe[2];
> >> +	struct io_timespec *iots;
> >> +	struct timespec64 ts;
> >> +	u32 tskey;
> >> +
> >> +	BUILD_BUG_ON(sizeof(struct io_uring_cqe) != sizeof(struct io_timespec));
> >> +
> >> +	if (!skb_get_tx_timestamp(skb, sk, &ts))
> >> +		return false;
> >> +
> >> +	tskey = serr->ee.ee_data;
> >> +
> >> +	cqe->user_data = 0;
> >> +	cqe->res = tskey;
> >> +	cqe->flags = IORING_CQE_F_MORE;
> >> +	cqe->flags |= (u32)serr->ee.ee_info << IORING_CQE_BUFFER_SHIFT;
> >> +
> >> +	iots = (struct io_timespec *)&cqe[1];
> >> +	iots->tv_sec = ts.tv_sec;
> >> +	iots->tv_nsec = ts.tv_nsec;
> > 
> > skb_get_tx_timestamp loses the information whether this is a
> > software or a hardware timestamp. Is that loss problematic?
> > 
> > If a process only requests one type of timestamp, it will not be.
> > 
> > But when requesting both (SOF_TIMESTAMPING_OPT_TX_SWHW) this per cqe
> > annotation may be necessary.
> 
> skb_has_tx_timestamp() helper has clear priority of software timestamp,
> if enabled for the socket. Looks like SOF_TIMESTAMPING_OPT_TX_SWHW case
> won't produce both timestamps with the current implementation. Am I
> missing something?

The point of that option is to request both SW and HW tx
timestamps. Before that option, the SW timestamp was suppressed if a
HW timestamp was pending.

The io_uring API as currently written will again return only one
timestamp, but this time the SW one.

If the user explicitly sets SOF_TIMESTAMPING_OPT_TX_SWHW, they will
want both, and want to be able to disambiguate them.