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 597717E for ; Mon, 8 May 2023 06:28:16 +0000 (UTC) Received: from out30-99.freemail.mail.aliyun.com (out30-99.freemail.mail.aliyun.com [115.124.30.99]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 18D7330E8; Sun, 7 May 2023 23:28:13 -0700 (PDT) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R191e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046056;MF=xuanzhuo@linux.alibaba.com;NM=1;PH=DS;RN=13;SR=0;TI=SMTPD_---0Vi.ayqL_1683527290; Received: from localhost(mailfrom:xuanzhuo@linux.alibaba.com fp:SMTPD_---0Vi.ayqL_1683527290) by smtp.aliyun-inc.com; Mon, 08 May 2023 14:28:11 +0800 Message-ID: <1683526688.7492425-1-xuanzhuo@linux.alibaba.com> Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500 Date: Mon, 8 May 2023 14:18:08 +0800 From: Xuan Zhuo To: "Michael S. Tsirkin" Cc: 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 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> In-Reply-To: <20230508020953-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-9.9 required=5.0 tests=BAYES_00, ENV_AND_HDR_SPF_MATCH,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_PASS, T_SCC_BODY_TEXT_LINE,UNPARSEABLE_RELAY,USER_IN_DEF_SPF_WL 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 X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: On Mon, 8 May 2023 02:15:46 -0400, "Michael S. Tsirkin" wr= ote: > On Mon, May 08, 2023 at 10:01:59AM +0800, Xuan Zhuo wrote: > > On Sun, 7 May 2023 04:58:58 -0400, "Michael S. Tsirkin" wrote: > > > On Sat, May 06, 2023 at 04:56:35PM +0800, Hao Chen wrote: > > > > > > > > > > > > =E5=9C=A8 2023/5/6 10:50, Xuan Zhuo =E5=86=99=E9=81=93: > > > > > On Sat, 6 May 2023 10:15:29 +0800, Hao Chen w= rote: > > > > > > When VIRTIO_NET_F_MTU(3) Device maximum MTU reporting is suppor= ted. > > > > > > If offered by the device, device advises driver about the value= of its > > > > > > maximum MTU. If negotiated, the driver uses mtu as the maximum > > > > > > MTU value. But there the driver also uses it as default mtu, > > > > > > some devices may have a maximum MTU greater than 1500, this may > > > > > > cause some large packages to be discarded, > > > > > > > > > > You mean tx packet? > > > > Yes. > > > > > > > > > > If yes, I do not think this is the problem of driver. > > > > > > > > > > Maybe you should give more details about the discard. > > > > > > > > > In the current code, if the maximum MTU supported by the virtio net= hardware > > > > is 9000, the default MTU of the virtio net driver will also be set = to 9000. > > > > When sending packets through "ping -s 5000", if the peer router doe= s not > > > > support negotiating a path MTU through ICMP packets, the packets wi= ll be > > > > discarded. If the peer router supports negotiating path mtu through= ICMP > > > > packets, the host side will perform packet sharding processing base= d on the > > > > negotiated path mtu, which is generally within 1500. > > > > This is not a bugfix patch, I think setting the default mtu to with= in 1500 > > > > would be more suitable here.Thanks. > > > > > > I don't think VIRTIO_NET_F_MTU is appropriate for support for jumbo p= ackets. > > > The spec says: > > > The device MUST forward transmitted packets of up to mtu (plus low l= evel ethernet header length) size with > > > gso_type NONE or ECN, and do so without fragmentation, after VIRTIO_= NET_F_MTU has been success- > > > fully negotiated. > > > VIRTIO_NET_F_MTU has been designed for all kind of tunneling devices, > > > and this is why we set mtu to max by default. > > > > > > For things like jumbo frames where MTU might or might not be availabl= e, > > > a new feature would be more appropriate. > > > > > > So for jumbo frame, what is the problem? > > > > We are trying to do this. @Heng > > > > Thanks. > > It is not a problem as such. But VIRTIO_NET_F_MTU will set the > default MTU not just the maximum one, because spec seems to > say it can. I see. In the case of Jumbo Frame, we also hope that the driver will set the defau= lt directly to the max mtu. Just like what you said "Bigger packets =3D better performance." 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. Of course, the scene that this patch wants to handle does exist, but I have never thought that this is a problem= at the driver level. Thanks. > > > > > > > > > > > > > so I changed the MTU to a more > > > > > > general 1500 when 'Device maximum MTU' bigger than 1500. > > > > > > > > > > > > Signed-off-by: Hao Chen > > > > > > --- > > > > > > drivers/net/virtio_net.c | 5 ++++- > > > > > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > > > > > > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > > > > > > index 8d8038538fc4..e71c7d1b5f29 100644 > > > > > > --- a/drivers/net/virtio_net.c > > > > > > +++ b/drivers/net/virtio_net.c > > > > > > @@ -4040,7 +4040,10 @@ static int virtnet_probe(struct virtio_d= evice *vdev) > > > > > > goto free; > > > > > > } > > > > > > > > > > > > - dev->mtu =3D mtu; > > > > > > + if (mtu > 1500) > > > > > > > > > > s/1500/ETH_DATA_LEN/ > > > > > > > > > > Thanks. > > > > > > > > > > > + dev->mtu =3D 1500; > > > > > > + else > > > > > > + dev->mtu =3D mtu; > > > > > > dev->max_mtu =3D mtu; > > > > > > } > > > > > > > > > > > > -- > > > > > > 2.27.0 > > > > > > > > > >