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

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Date:   Tue, 2 Feb 2021 08:52:21 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Arjun Roy <arjunroy@...gle.com>
Cc:     David Ahern <dsahern@...il.com>, Jakub Kicinski <kuba@...nel.org>,
        Arjun Roy <arjunroy.kdev@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [net-next v2 2/2] tcp: Add receive timestamp support for receive
 zerocopy.

On Mon, Feb 01, 2021 at 06:20:23PM -0800, Arjun Roy wrote:
> On Mon, Feb 1, 2021 at 6:06 PM David Ahern <dsahern@...il.com> wrote:
> >
> > On 1/24/21 11:15 PM, Leon Romanovsky wrote:
> > > 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 :).
> > >
> >
> > agreed.
> >
> > Arjun: would you mind following up with a patch to make this hole
> > explicit and usable for the next extension? Thanks,
>
> Will do.

Please pay attention that all "in" and "out" fields that marked as reserved
should be zeroed and kernel must check "in" field to ensure future compatibility.

Thanks

>
> -Arjun