From owner-svn-src-stable@FreeBSD.ORG Sat Dec 29 10:44:59 2012 Return-Path: Delivered-To: svn-src-stable@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 938E3C02; Sat, 29 Dec 2012 10:44:59 +0000 (UTC) (envelope-from rwatson@freebsd.org) Received: from cyrus.watson.org (cyrus.watson.org [65.122.17.42]) by mx1.freebsd.org (Postfix) with ESMTP id 5D92E8FC08; Sat, 29 Dec 2012 10:44:59 +0000 (UTC) Received: from [192.168.2.119] (host86-129-88-139.range86-129.btcentralplus.com [86.129.88.139]) by cyrus.watson.org (Postfix) with ESMTPSA id 42E5946B09; Sat, 29 Dec 2012 05:44:51 -0500 (EST) Subject: Re: svn commit: r244663 - stable/9 Mime-Version: 1.0 (Apple Message framework v1283) Content-Type: text/plain; charset=iso-8859-1 From: "Robert N. M. Watson" In-Reply-To: Date: Sat, 29 Dec 2012 10:44:43 +0000 Content-Transfer-Encoding: quoted-printable Message-Id: <00E4FFFA-8ADB-4D43-B977-3834C48133E4@freebsd.org> References: <201212241422.qBOEMrcF021632@svn.freebsd.org> <50D8B533.8080507@mu.org> <20121225104422.GB53644@kib.kiev.ua> To: Adrian Chadd X-Mailer: Apple Mail (2.1283) Cc: src-committers@freebsd.org, svn-src-stable@freebsd.org, svn-src-all@freebsd.org, Alfred Perlstein , svn-src-stable-9@freebsd.org, Konstantin Belousov X-BeenThere: svn-src-stable@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: SVN commit messages for all the -stable branches of the src tree List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 29 Dec 2012 10:44:59 -0000 On 29 Dec 2012, at 06:40, Adrian Chadd wrote: > There's likely a bunch of companies/users that would love things to > not change during a stable branch and there's likely a bunch of > companies/users that would hate things being immutable during a stable > branch. >=20 > There's never been a formal "kernel ABI stuff in stable shouldn't > break" or not, but as far as I was aware, the unofficial method was > "discuss on -stable or -arch to see whether it's worth the break, then > break it if needed, or not-break it and add a dirty hack for that > branch" if not. >=20 > This is why things like vimage/vnet were so dirty in the backports, if > you remember. Julian and others made a specific attempt _not_ to break > KBI when backporting the feature. >=20 > So, regardless of whether we should or shouldn't break things, a more > thorough discussion would've been nice. Adrian: The standing consensus is that we try not to break certain classes of = device drivers, not that we don't ever change any kernel interfaces. The = reason is that we don't have a formal definition of "public" and do not = wish to use the definition "all definitions in all header files" or "all = symbols ever linked by any module" -- that definition would prevent = almost any change to the kernel in -STABLE branches at all. The reason = VIMAGE/MRT/etc had to be done with great caution is that they directly = affected network device drivers, which are a category of module which we = have decided we do want to try to support in external binary form. The = other major category is binary storage drivers. When we talked to various VFS maintainers, looked at the past change = history there, and looked at the set of third-party file systems = (especially, those we could see in ports), the consensus there was that = it was too difficult to define a stable VFS KPI and KBI for third-party = modules. In particular, there appear to be at most one or two in ports = at any given moment, and quick analyses of them suggested that their = kernel feature dependency footprint was far more than just "vnode = operations". KPIs and KBIs have benefits and downsides: we need to consider them as a = tradeoff space, and not an absolute, and use them where they have = significant payoff. Especially as we don't have formal tools for = reasoning about or testing them. Robert=