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Tsirkin" To: Jason Wang Subject: Re: [PATCH net] virtio-net: enable big mode correctly Message-ID: <20211125015532-mutt-send-email-mst@kernel.org> References: <20211125060547.11961-1-jasowang@redhat.com> MIME-Version: 1.0 In-Reply-To: <20211125060547.11961-1-jasowang@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: netdev@vger.kernel.org, Eli Cohen , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" On Thu, Nov 25, 2021 at 02:05:47PM +0800, Jason Wang wrote: > When VIRTIO_NET_F_MTU feature is not negotiated, we assume a very > large max_mtu. In this case, using small packet mode is not correct > since it may breaks the networking when MTU is grater than > ETH_DATA_LEN. > > To have a quick fix, simply enable the big packet mode when > VIRTIO_NET_F_MTU is not negotiated. This will slow down dpdk hosts which disable mergeable buffers and send standard MTU sized packets. > We can do optimization on top. I don't think it works like this, increasing mtu from guest >4k never worked, we can't regress everyone's performance with a promise to maybe sometime bring it back. > Reported-by: Eli Cohen > Cc: Eli Cohen > Signed-off-by: Jason Wang > > --- > drivers/net/virtio_net.c | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > index 7c43bfc1ce44..83ae3ef5eb11 100644 > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -3200,11 +3200,12 @@ static int virtnet_probe(struct virtio_device *vdev) > dev->mtu = mtu; > dev->max_mtu = mtu; > > - /* TODO: size buffers correctly in this case. */ > - if (dev->mtu > ETH_DATA_LEN) > - vi->big_packets = true; > } > > + /* TODO: size buffers correctly in this case. */ > + if (dev->max_mtu > ETH_DATA_LEN) > + vi->big_packets = true; > + > if (vi->any_header_sg) > dev->needed_headroom = vi->hdr_len; > > -- > 2.25.1 _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization