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Tsirkin" To: Xuan Zhuo Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500 Message-ID: <20230508024147-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> MIME-Version: 1.0 In-Reply-To: <1683526688.7492425-1-xuanzhuo@linux.alibaba.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: zy@yusur.tech, Hao Chen , "open list:NETWORKING DRIVERS" , open list , "open list:VIRTIO CORE AND NET DRIVERS" , Eric Dumazet , hengqi@linux.alibaba.com, huangml@yusur.tech, Jakub Kicinski , Paolo Abeni , "David S. 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([31.187.78.15]) by smtp.gmail.com with ESMTPSA id k1-20020a05600c1c8100b003f4283f5c1bsm456649wms.2.2023.05.07.23.43.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 07 May 2023 23:43:29 -0700 (PDT) Date: Mon, 8 May 2023 02:43:24 -0400 From: "Michael S. Tsirkin" To: Xuan Zhuo 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 Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500 Message-ID: <20230508024147-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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1683526688.7492425-1-xuanzhuo@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 08, 2023 at 02:18:08PM +0800, Xuan Zhuo wrote: > On Mon, 8 May 2023 02:15:46 -0400, "Michael S. Tsirkin" wrote: > > 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: > > > > > > > > > > > > > > > 在 2023/5/6 10:50, Xuan Zhuo 写道: > > > > > > On Sat, 6 May 2023 10:15:29 +0800, Hao Chen wrote: > > > > > > > When VIRTIO_NET_F_MTU(3) Device maximum MTU reporting is supported. > > > > > > > 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 does not > > > > > support negotiating a path MTU through ICMP packets, the packets will be > > > > > discarded. If the peer router supports negotiating path mtu through ICMP > > > > > packets, the host side will perform packet sharding processing based on the > > > > > negotiated path mtu, which is generally within 1500. > > > > > This is not a bugfix patch, I think setting the default mtu to within 1500 > > > > > would be more suitable here.Thanks. > > > > > > > > I don't think VIRTIO_NET_F_MTU is appropriate for support for jumbo packets. > > > > The spec says: > > > > The device MUST forward transmitted packets of up to mtu (plus low level 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 available, > > > > 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 default > directly to the max mtu. Just like what you said "Bigger packets = 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. When other devices on the same LAN have mtu set to 1500 and won't accept bigger packets. > 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_device *vdev) > > > > > > > goto free; > > > > > > > } > > > > > > > > > > > > > > - dev->mtu = mtu; > > > > > > > + if (mtu > 1500) > > > > > > > > > > > > s/1500/ETH_DATA_LEN/ > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > + dev->mtu = 1500; > > > > > > > + else > > > > > > > + dev->mtu = mtu; > > > > > > > dev->max_mtu = mtu; > > > > > > > } > > > > > > > > > > > > > > -- > > > > > > > 2.27.0 > > > > > > > > > > > > >