From owner-freebsd-current@FreeBSD.ORG  Tue Sep  9 10:13:46 2014
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: current@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115])
 (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id 60098C91;
 Tue,  9 Sep 2014 10:13:46 +0000 (UTC)
Received: from mail-lb0-x22a.google.com (mail-lb0-x22a.google.com
 [IPv6:2a00:1450:4010:c04::22a])
 (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits))
 (Client CN "smtp.gmail.com",
 Issuer "Google Internet Authority G2" (verified OK))
 by mx1.freebsd.org (Postfix) with ESMTPS id B17DCEB6;
 Tue,  9 Sep 2014 10:13:45 +0000 (UTC)
Received: by mail-lb0-f170.google.com with SMTP id u10so1992953lbd.1
 for <multiple recipients>; Tue, 09 Sep 2014 03:13:43 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113;
 h=mime-version:sender:date:message-id:subject:from:to:cc:content-type;
 bh=m2FKsg6E121Dg+TkIA7401URhE+G1ICTGfDLYm2Buqo=;
 b=SNEu+BJ5qcXOPLxlfk2A8hN/x1wz4D1qtRrPV3V5usjiFS0rBN635wmnSkmhgpDzr3
 zvdOHPcUJoATb2L4bAXXJs+UU0Q80Dk4vmZNq2h02GhocsnG7KZmKn+WXE2hqadAms1H
 CYHJt3KoLu9eFvf8G8X1zL13olsrk5gw+mAyqT6b5v2HOmdYwdFRq28R5k58k+0NMqcT
 rVMmcNwX+3EB6SHROTK9opDa/9OqITLSjmUbO7M+fwWJPxc9rp3tnKCdFkzGSFXIIYN1
 1yxBn09f+qS+BpJRgJdRoeTYnXTfUdMzsNqXBPVAUtEV9qd5tOA6afJFJMzR03GIpbNZ
 S+iQ==
MIME-Version: 1.0
X-Received: by 10.152.42.136 with SMTP id o8mr3845300lal.71.1410257622956;
 Tue, 09 Sep 2014 03:13:42 -0700 (PDT)
Sender: rizzo.unipi@gmail.com
Received: by 10.114.26.37 with HTTP; Tue, 9 Sep 2014 03:13:42 -0700 (PDT)
Date: Tue, 9 Sep 2014 12:13:42 +0200
X-Google-Sender-Auth: HDOzditjs0cTwLo34WbzWQtqKIQ
Message-ID: <CA+hQ2+iLGH+h6asbQSES0P134_5KUgnt8Ve__UQgf9cwu_97eA@mail.gmail.com>
Subject: RFC: please put back spare fields in struct ifnet (removed in svn
 270870)
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: FreeBSD Current <current@freebsd.org>, Gleb Smirnoff <glebius@freebsd.org>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
X-Content-Filtered-By: Mailman/MimeDel 2.1.18-1
Cc: George Neville-Neil <gnn@neville-neil.com>,
 Stefano Garzarella <stefanogarzarella@gmail.com>
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.18-1
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
 <freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-current>, 
 <mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current/>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
 <mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Tue, 09 Sep 2014 10:13:46 -0000

hi,
sorry if i just noticed it recently.

svn 270870 removed all the if_*spare fields in struct ifnet.
They are replaced with the following comment

/*
 * Spare fields to be added before branching a stable branch, so
 * that structure can be enhanced without changing the kernel
 * binary interface.
 */

=E2=80=8Bwhich leaves me a bit unhappy.
Having a stable ABI is useful not only for stable branches,
but also (I would say even more) with head, so people can
run experimental code with limited modifications to the sources.

Cases in point:
- we used one spare field extensively when experimenting
  with netmap, and being able to just build a module without having
  to recompile the whole kernel was a big win.
- we are developing some software GSO and again it was great to have
  the spares in the tcpcb and in the ifnet so we could limit
  modifications to headers used by multiple sources.

I would kindly suggest to put the spares back.
I can't see how they can possibly harm.

thanks
luigi