Michael S. Tsirkin
2021-Nov-25 07:00 UTC
[PATCH net] virtio-net: enable big mode correctly
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 <elic at nvidia.com> > Cc: Eli Cohen <elic at nvidia.com> > Signed-off-by: Jason Wang <jasowang at redhat.com> > > --- > 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
On Thu, Nov 25, 2021 at 3:00 PM Michael S. Tsirkin <mst at redhat.com> wrote:> > 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,Looking at add_recvbuf_small() it's actually GOOD_PACKET_LEN if I was not wrong.> we can't regress everyone's > performance with a promise to maybe sometime bring it back.So consider it never work before I wonder if we can assume a 1500 as max_mtu value instead of simply using MAX_MTU? Thanks> > > Reported-by: Eli Cohen <elic at nvidia.com> > > Cc: Eli Cohen <elic at nvidia.com> > > Signed-off-by: Jason Wang <jasowang at redhat.com> > > > > --- > > 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 >