From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.7 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 56D96C3A5A6 for ; Sun, 22 Sep 2019 22:46:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 042A020830 for ; Sun, 22 Sep 2019 22:46:37 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="eVj4EWZI" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389654AbfIVWqe (ORCPT ); Sun, 22 Sep 2019 18:46:34 -0400 Received: from mail-vk1-f194.google.com ([209.85.221.194]:45609 "EHLO mail-vk1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389439AbfIVWqe (ORCPT ); Sun, 22 Sep 2019 18:46:34 -0400 Received: by mail-vk1-f194.google.com with SMTP id u192so2584113vkb.12; Sun, 22 Sep 2019 15:46:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=rVJRFaJE5AEAesOtuJI9NH1xVfCZ4xaAZV4mZPfTXDI=; b=eVj4EWZImuOMCwXehKHEqP97MAgGkS1KslAMdmp/aFVLlAwjvoNI+ffLxG+uvOrg8w /TeMpbJAt4zTxI5bv8sAxcSrwKoGIb/QnYNf6PcIkleSXYTh1hJWpKdEnjna5RM+m/SY td1xa0hcFkWI88gzZyQsiRT6cp6KAnUnf57QGixWPPQxlNnUuH/4O0TP1IxMYmh/37WM 3aqUB+FNbpLdpUPO4eEPh07LDF4BWrESlbWHRqScHil4GJPhzcuPeZWA9x+2ZUU2s1F8 m7g8qwqJuKiXSFvp7hkczxj6rilsT/DB7ZgEI2R9q968GpdOeKY7oR9KTK72PiK8LDni I/Hg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=rVJRFaJE5AEAesOtuJI9NH1xVfCZ4xaAZV4mZPfTXDI=; b=e70XTmuORYJDuLb1+Rz40npEwYP++o7H17vnA/WgOuIo0jZk87sei5KLLQmvsUBk9w H0dPPkrpED4ZCOogjxIavIKcdxqUceH5H6U0Fdo/STU2DW4UcOvLSMxxF/r3hg0JBfBP acOlhH4Lqu4VVqKvKQyY0tTgV7Yjl7iEdhW/1c4tBV/09lMZd8zlkjaxZXbUIMFnmNj7 PsuzwTToUWYC/gd7f/YkKKRtWB95AY6Pz8vR2wrvF9Bsejv7b/vzH2llRpssp7ma6UAK i9tH2QFtoLm0/wgfu2CztK4p7PIPaD4kqam6ypUBFs97r8GcnFH5V4K5rD2kc6E/eWpA cVVQ== X-Gm-Message-State: APjAAAV34JnWsc0/qzai/YXDMYBX4lYChSvlGfHCH5EZ4EGLiL/fl93M GEXW4i43UatO00WTqrowiPoSKaYMmJsty9uWho4= X-Google-Smtp-Source: APXvYqxOFG9/xZZVxAoYT/1HFqcEGT5aNLeW3buObGdou+/30q6Ni0mz/dpmFMtaY/qKPwvP0N3bpVUKouHZa6knUL4= X-Received: by 2002:ac5:c4d1:: with SMTP id a17mr13574798vkl.57.1569192390979; Sun, 22 Sep 2019 15:46:30 -0700 (PDT) MIME-Version: 1.0 References: <20190920185843.4096-1-matthew.cover@stackpath.com> <20190922080326-mutt-send-email-mst@kernel.org> <20190922162546-mutt-send-email-mst@kernel.org> In-Reply-To: From: Matt Cover Date: Sun, 22 Sep 2019 15:46:19 -0700 Message-ID: Subject: Re: [PATCH net-next] tuntap: Fallback to automq on TUNSETSTEERINGEBPF prog negative return To: "Michael S. Tsirkin" Cc: davem@davemloft.net, ast@kernel.org, daniel@iogearbox.net, kafai@fb.com, songliubraving@fb.com, yhs@fb.com, Jason Wang , Eric Dumazet , Stanislav Fomichev , Matthew Cover , mail@timurcelik.de, pabeni@redhat.com, Nicolas Dichtel , wangli39@baidu.com, lifei.shirley@bytedance.com, tglx@linutronix.de, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Sep 22, 2019 at 3:30 PM Matt Cover wrote: > > On Sun, Sep 22, 2019 at 1:36 PM Michael S. Tsirkin wrote: > > > > On Sun, Sep 22, 2019 at 10:43:19AM -0700, Matt Cover wrote: > > > On Sun, Sep 22, 2019 at 5:37 AM Michael S. Tsirkin wrote: > > > > > > > > On Fri, Sep 20, 2019 at 11:58:43AM -0700, Matthew Cover wrote: > > > > > Treat a negative return from a TUNSETSTEERINGEBPF bpf prog as a signal > > > > > to fallback to tun_automq_select_queue() for tx queue selection. > > > > > > > > > > Compilation of this exact patch was tested. > > > > > > > > > > For functional testing 3 additional printk()s were added. > > > > > > > > > > Functional testing results (on 2 txq tap device): > > > > > > > > > > [Fri Sep 20 18:33:27 2019] ========== tun no prog ========== > > > > > [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '-1' > > > > > [Fri Sep 20 18:33:27 2019] tuntap: tun_automq_select_queue() ran > > > > > [Fri Sep 20 18:33:27 2019] ========== tun prog -1 ========== > > > > > [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '-1' > > > > > [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '-1' > > > > > [Fri Sep 20 18:33:27 2019] tuntap: tun_automq_select_queue() ran > > > > > [Fri Sep 20 18:33:27 2019] ========== tun prog 0 ========== > > > > > [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '0' > > > > > [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '0' > > > > > [Fri Sep 20 18:33:27 2019] ========== tun prog 1 ========== > > > > > [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '1' > > > > > [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '1' > > > > > [Fri Sep 20 18:33:27 2019] ========== tun prog 2 ========== > > > > > [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '2' > > > > > [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '0' > > > > > > > > > > Signed-off-by: Matthew Cover > > > > > > > > > > > > Could you add a bit more motivation data here? > > > > > > Thank you for these questions Michael. > > > > > > I'll plan on adding the below information to the > > > commit message and submitting a v2 of this patch > > > when net-next reopens. In the meantime, it would > > > be very helpful to know if these answers address > > > some of your concerns. > > > > > > > 1. why is this a good idea > > > > > > This change allows TUNSETSTEERINGEBPF progs to > > > do any of the following. > > > 1. implement queue selection for a subset of > > > traffic (e.g. special queue selection logic > > > for ipv4, but return negative and use the > > > default automq logic for ipv6) > > > 2. determine there isn't sufficient information > > > to do proper queue selection; return > > > negative and use the default automq logic > > > for the unknown > > > 3. implement a noop prog (e.g. do > > > bpf_trace_printk() then return negative and > > > use the default automq logic for everything) > > > > > > > 2. how do we know existing userspace does not rely on existing behaviour > > > > > > Prior to this change a negative return from a > > > TUNSETSTEERINGEBPF prog would have been cast > > > into a u16 and traversed netdev_cap_txqueue(). > > > > > > In most cases netdev_cap_txqueue() would have > > > found this value to exceed real_num_tx_queues > > > and queue_index would be updated to 0. > > > > > > It is possible that a TUNSETSTEERINGEBPF prog > > > return a negative value which when cast into a > > > u16 results in a positive queue_index less than > > > real_num_tx_queues. For example, on x86_64, a > > > return value of -65535 results in a queue_index > > > of 1; which is a valid queue for any multiqueue > > > device. > > > > > > It seems unlikely, however as stated above is > > > unfortunately possible, that existing > > > TUNSETSTEERINGEBPF programs would choose to > > > return a negative value rather than return the > > > positive value which holds the same meaning. > > > > > > It seems more likely that future > > > TUNSETSTEERINGEBPF programs would leverage a > > > negative return and potentially be loaded into > > > a kernel with the old behavior. > > > > OK if we are returning a special > > value, shouldn't we limit it? How about a special > > value with this meaning? > > If we are changing an ABI let's at least make it > > extensible. > > > > A special value with this meaning sounds > good to me. I'll plan on adding a define > set to -1 to cause the fallback to automq. > > The way I was initially viewing the old > behavior was that returning negative was > undefined; it happened to have the > outcomes I walked through, but not > necessarily by design. > > In order to keep the new behavior > extensible, how should we state that a > negative return other than -1 is > undefined and therefore subject to > change. Is something like this > sufficient? > > Documentation/networking/tc-actions-env-rules.txt > > Additionally, what should the new > behavior implement when a negative other > than -1 is returned? I would like to have > it do the same thing as -1 for now, but > with the understanding that this behavior > is undefined. Does this sound reasonable? > > > > > 3. why doesn't userspace need a way to figure out whether it runs on a kernel with and > > > > without this patch > > > > > > There may be some value in exposing this fact > > > to the ebpf prog loader. What is the standard > > > practice here, a define? > > > > > > We'll need something at runtime - people move binaries between kernels > > without rebuilding then. An ioctl is one option. > > A sysfs attribute is another, an ethtool flag yet another. > > A combination of these is possible. > > > > And if we are doing this anyway, maybe let userspace select > > the new behaviour? This way we can stay compatible with old > > userspace... > > > > Understood. I'll look into adding an > ioctl to activate the new behavior. And > perhaps a method of checking which is > behavior is currently active (in case we > ever want to change the default, say > after some suitably long transition > period). > Unless of course we can simply state via documentation that any negative return for which a define doesn't exist is undefined behavior. In which case, there is no old vs new behavior and no need for an ioctl. Simply the understanding provided by the documentation. > > > > > > > > > > > > thanks, > > > > MST > > > > > > > > > --- > > > > > drivers/net/tun.c | 20 +++++++++++--------- > > > > > 1 file changed, 11 insertions(+), 9 deletions(-) > > > > > > > > > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c > > > > > index aab0be4..173d159 100644 > > > > > --- a/drivers/net/tun.c > > > > > +++ b/drivers/net/tun.c > > > > > @@ -583,35 +583,37 @@ static u16 tun_automq_select_queue(struct tun_struct *tun, struct sk_buff *skb) > > > > > return txq; > > > > > } > > > > > > > > > > -static u16 tun_ebpf_select_queue(struct tun_struct *tun, struct sk_buff *skb) > > > > > +static int tun_ebpf_select_queue(struct tun_struct *tun, struct sk_buff *skb) > > > > > { > > > > > struct tun_prog *prog; > > > > > u32 numqueues; > > > > > - u16 ret = 0; > > > > > + int ret = -1; > > > > > > > > > > numqueues = READ_ONCE(tun->numqueues); > > > > > if (!numqueues) > > > > > return 0; > > > > > > > > > > + rcu_read_lock(); > > > > > prog = rcu_dereference(tun->steering_prog); > > > > > if (prog) > > > > > ret = bpf_prog_run_clear_cb(prog->prog, skb); > > > > > + rcu_read_unlock(); > > > > > > > > > > - return ret % numqueues; > > > > > + if (ret >= 0) > > > > > + ret %= numqueues; > > > > > + > > > > > + return ret; > > > > > } > > > > > > > > > > static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb, > > > > > struct net_device *sb_dev) > > > > > { > > > > > struct tun_struct *tun = netdev_priv(dev); > > > > > - u16 ret; > > > > > + int ret; > > > > > > > > > > - rcu_read_lock(); > > > > > - if (rcu_dereference(tun->steering_prog)) > > > > > - ret = tun_ebpf_select_queue(tun, skb); > > > > > - else > > > > > + ret = tun_ebpf_select_queue(tun, skb); > > > > > + if (ret < 0) > > > > > ret = tun_automq_select_queue(tun, skb); > > > > > - rcu_read_unlock(); > > > > > > > > > > return ret; > > > > > } > > > > > -- > > > > > 1.8.3.1