From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3BBB023CD for ; Mon, 8 May 2023 16:25:54 +0000 (UTC) Received: from mail-pf1-x435.google.com (mail-pf1-x435.google.com [IPv6:2607:f8b0:4864:20::435]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 375003AB1 for ; Mon, 8 May 2023 09:25:51 -0700 (PDT) Received: by mail-pf1-x435.google.com with SMTP id d2e1a72fcca58-64384c6797eso3821015b3a.2 for ; Mon, 08 May 2023 09:25:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=networkplumber-org.20221208.gappssmtp.com; s=20221208; t=1683563150; x=1686155150; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:subject:cc:to:from:date:from:to:cc:subject:date :message-id:reply-to; bh=UMRspvRrnCiX2PGc3SiKh0fa94AnkU5LDoj0pGSvFHU=; b=0SDB6x2PHopDhbk1y+a0LH8gjtwXcG7OYClmL4E4hHEip2ovsG5tkhchFHrxTrrA9T UM7+QFe9A5ex3NK/62W8E0lJU5kX2Sve1UU5fuA2WwxrICcyYAnHmmD1hFERVABTGMjH dy4E1aj8COJMv+/RpDGzew2Aajd9ExACP2f9a1zsgM8H0XRRtRKp5GG7J4yqDiMqxldA BRtd54MZvy2wNOmWt4dXYX3k7OGf6v/O1W/IffhIu/N76qGRWhWcFsaxP3aumeb0PlNp Nwcqc8AsiKGSEWSGbxEF4mh/cvXYECcde7K5BUPQluuiFlDhwskEtpHL3Z/Sn609p1NU YWhQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1683563150; x=1686155150; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:subject:cc:to:from:date:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=UMRspvRrnCiX2PGc3SiKh0fa94AnkU5LDoj0pGSvFHU=; b=Muope/T1QhI6IiYZcFkh/82Zl96nhaHi9lbPVqKATLXycfZzNy5XL8S1niEP8mlXZW iqcG3mMs7M0OhTqqmd64JjVTKJbOkLUYCGsczsAlrEiKEK+3PCTqzxjxHmpbWHhblDsd 2gpESwnMMRg4kX39x1Ect2dsy073SMNJPDRLlSHWXmFr1oDyo7c3XtDQZgusj9BKy1n+ OctKFE5huksc5NYbmZuOZWfJVvWOQeXbfupsvpYCw/MbcREjE1M49llu+5YnfGgalDsf DCJLLAsag8UbOKeYNO/kuk/GSHXqP4J//NBG3lxn2cw1g5IGNGDpcAYzm49FbvtK5Rnm SELA== X-Gm-Message-State: AC+VfDysHlj2jPbbY4BpnFbZ770pNyIkTjptemfB/2tlwkKeuyJDYmmo CIBrCSfp44fsonYuo9dBWE6ak4vdFeBz4RPE5gqUFg== X-Google-Smtp-Source: ACHHUZ6xW6/XcFMUPs+QzLs8J3iNk575OvzS5PxyJ5II6bUkrr8idBhdP+G4wYSZ5Ul/qBGSe63n/Q== X-Received: by 2002:a05:6a00:2e1b:b0:63d:6744:8caf with SMTP id fc27-20020a056a002e1b00b0063d67448cafmr13482081pfb.26.1683563150656; Mon, 08 May 2023 09:25:50 -0700 (PDT) Received: from hermes.local (204-195-120-218.wavecable.com. [204.195.120.218]) by smtp.gmail.com with ESMTPSA id s19-20020aa78d53000000b0063b86aff031sm164331pfe.108.2023.05.08.09.25.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 08 May 2023 09:25:50 -0700 (PDT) Date: Mon, 8 May 2023 09:25:48 -0700 From: Stephen Hemminger To: "Michael S. Tsirkin" Cc: Xuan Zhuo , huangml@yusur.tech, zy@yusur.tech, Jason Wang , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , "open list:VIRTIO CORE AND NET DRIVERS" , "open list:NETWORKING DRIVERS" , open list , Hao Chen , hengqi@linux.alibaba.com Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500 Message-ID: <20230508092548.5fc8f078@hermes.local> In-Reply-To: <20230508062928-mutt-send-email-mst@kernel.org> References: <20230506021529.396812-1-chenh@yusur.tech> <1683341417.0965195-4-xuanzhuo@linux.alibaba.com> <07b6b325-9a15-222f-e618-d149b57cbac2@yusur.tech> <20230507045627-mutt-send-email-mst@kernel.org> <1683511319.099806-2-xuanzhuo@linux.alibaba.com> <20230508020953-mutt-send-email-mst@kernel.org> <1683526688.7492425-1-xuanzhuo@linux.alibaba.com> <20230508024147-mutt-send-email-mst@kernel.org> <1683531716.238961-1-xuanzhuo@linux.alibaba.com> <20230508062928-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS, T_SCC_BODY_TEXT_LINE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net On Mon, 8 May 2023 06:30:07 -0400 "Michael S. Tsirkin" wrote: > > > > I don't know, in any scenario, when the hardware supports a large mtu, but we do > > > > not want the user to use it by default. > > > > > > When other devices on the same LAN have mtu set to 1500 and > > > won't accept bigger packets. > > > > So, that depends on pmtu/tcp-probe-mtu. > > > > If the os without pmtu/tcp-probe-mtu has a bigger mtu, then it's big packet > > will lost. > > > > Thanks. > > > > pmtu is designed for routing. LAN is supposed to be configured with > a consistent MTU. Virtio is often used with bridging or macvlan which can't support PMTU. PMTU only works when forwarding at layer 3 (ie routing) where there is a IP address to send the ICMP response. If doing L2 forwarding, the only thin the bridge can do is drop the packet. TCP cab recover but detecting an MTU blackhole requires retransmissions.