Subject: Re: [net-next v2 2/2] tcp: Add receive timestamp support for receive zerocopy.

From: Leon Romanovsky <[email-masked]>
Date: Mon, 25 Jan 2021 08:15:08 +0200

On Fri, Jan 22, 2021 at 10:55:45PM -0700, David Ahern wrote:
> On 1/22/21 9:07 PM, Jakub Kicinski wrote:
> > On Wed, 20 Jan 2021 16:41:48 -0800 Arjun Roy wrote:
> >> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> >> index 768e93bd5b51..b216270105af 100644
> >> --- a/include/uapi/linux/tcp.h
> >> +++ b/include/uapi/linux/tcp.h
> >> @@ -353,5 +353,9 @@ struct tcp_zerocopy_receive {
> >>  	__u64 copybuf_address;	/* in: copybuf address (small reads) */
> >>  	__s32 copybuf_len; /* in/out: copybuf bytes avail/used or error */
> >>  	__u32 flags; /* in: flags */
> >> +	__u64 msg_control; /* ancillary data */
> >> +	__u64 msg_controllen;
> >> +	__u32 msg_flags;
> >> +	/* __u32 hole;  Next we must add >1 u32 otherwise length checks fail. */
> >
> > Well, let's hope nobody steps on this landmine.. :)
> >
>
> Past suggestions were made to use anonymous declarations - e.g., __u32
> :32; - as a way of reserving the space for future use. That or declare
> '__u32 resvd', check that it must be 0 and makes it available for later
> (either directly or with a union).

This is the schema (reserved field without union) used by the RDMA UAPIs from
the beginning (>20 years already) and it works like a charm.

Highly recommend :).

Thanks