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[2620:137:e000::1:20]) by mx.google.com with ESMTP id x1-20020a170902ec8100b001ac4e2d2b58si4000678plg.112.2023.05.07.02.55.00; Sun, 07 May 2023 02:55:14 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; dkim=pass header.i=@infradead.org header.s=desiato.20200630 header.b=Dzh1GZYd; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231396AbjEGJe1 (ORCPT + 99 others); Sun, 7 May 2023 05:34:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47544 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231322AbjEGJeY (ORCPT ); Sun, 7 May 2023 05:34:24 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1F7F511D86; Sun, 7 May 2023 02:34:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=Content-Transfer-Encoding:Content-Type :MIME-Version:Message-ID:References:In-Reply-To:Subject:CC:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=8O7iwhKUEtaC+yGWAkkZj8ftLvq2X4vGALr4kxZfwTo=; b=Dzh1GZYdDsBqW+xGH6mcESgJJe xYVa9SrJxeDOAI7OzrzmB8R6QChr/xqrADHHkpL26MjtHI5OoH14obPfYqzwRB9ZeELDVO9+Et36+ Q6rZJeNW7tVCA5t+LCDxQhxOUk8Imn/C8iAWvCyLGMwVAoaxtV4QWXHJJepwbkACiBJxIfV4MF5D2 am7J2sstO0SlCCs90gz1hMT5MYqDp6p0cq0N8RCKHYsxQb0jhuhBd+0HMXuu8zzngGMSiG3cqtzts 7yb/70jqAnEZeF8HdZv+F+ErLMrjdAsTVaoAVfqFs2vn901A9gWXOFntuu7Nl/XXCGplhN37cxHjK AnMaVepQ==; Received: from [2a00:23ee:19f0:787b:4aaa:b478:2a2b:a116] (helo=[IPv6:::1]) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1pvak0-004P3l-0G; Sun, 07 May 2023 09:33:41 +0000 Date: Sun, 07 May 2023 10:31:34 +0100 From: David Woodhouse To: Hao Chen , Xuan Zhuo CC: huangml@yusur.tech, zy@yusur.tech, "Michael S. Tsirkin" , Jason Wang , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , "open list:VIRTIO CORE AND NET DRIVERS" , "open list:NETWORKING DRIVERS" , open list Subject: =?US-ASCII?Q?Re=3A_=5BPATCH=5D_virtio=5Fnet=3A_set_default_mtu_to_15?= =?US-ASCII?Q?00_when_=27Device_maximum_MTU=27_bigger_than_1500?= User-Agent: K-9 Mail for Android In-Reply-To: <07b6b325-9a15-222f-e618-d149b57cbac2@yusur.tech> References: <20230506021529.396812-1-chenh@yusur.tech> <1683341417.0965195-4-xuanzhuo@linux.alibaba.com> <07b6b325-9a15-222f-e618-d149b57cbac2@yusur.tech> Message-ID: <49455AA8-0606-447E-B455-F905EA184E42@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,SPF_HELO_NONE, SPF_NONE,T_SCC_BODY_TEXT_LINE,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6 May 2023 09:56:35 BST, Hao Chen wrote: >In the current code, if the maximum MTU supported by the virtio net hardw= are is 9000, the default MTU of the virtio net driver will also be set to 9= 000=2E When sending packets through "ping -s 5000", if the peer router does= not support negotiating a path MTU through ICMP packets, the packets will = be discarded=2E That router is just plain broken, and it's going to break all kinds of tra= ffic=2E Hacking the virtio-net MTU is only a partial workaround=2E Surely the correct fix here is to apply percussive education to whatever i= diot thought it was OK to block ICMP=2E Not to hack the default MTU of one = device to the lowest common denominator=2E