Re: [PATCH net] virtio-net: enable big mode correctly
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2021-11-25 07:02:41
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lkml, netdev
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.
quoted hunk ↗ jump to hunk
Reported-by: Eli Cohen <redacted> Cc: Eli Cohen <redacted> Signed-off-by: Jason Wang <jasowang@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
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