[net PATCH 1/2] virtio_net: cap mtu when XDP programs are running

[John Fastabend]

XDP programs can not consume multiple pages so we cap the MTU to
avoid this case. Virtio-net however only checks the MTU at XDP
program load and does not block MTU changes after the program
has loaded.

This patch sets/clears the max_mtu value at XDP load/unload time.

Signed-off-by: John Fastabend <john.r.fastab...@intel.com>
---
 drivers/net/virtio_net.c |   26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4a10500..261103d9 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1699,11 +1699,28 @@ static void virtnet_init_settings(struct net_device 
*dev)
        .set_settings = virtnet_set_settings,
 };
 
+#define MIN_MTU ETH_MIN_MTU
+#define MAX_MTU ETH_MAX_MTU
+
+static unsigned long int virtnet_xdp_mtu(struct bpf_prog *prog,
+                                        struct virtnet_info *vi)
+{
+       if (!prog && virtio_has_feature(vi->vdev, VIRTIO_NET_F_MTU))
+               return virtio_cread16(vi->vdev,
+                                     offsetof(struct virtio_net_config, mtu));
+       else if (!prog)
+               return ETH_MAX_MTU;
+       else if (vi->mergeable_rx_bufs)
+               return PAGE_SIZE - sizeof(struct padded_vnet_hdr);
+       else
+               return GOOD_PACKET_LEN;
+}
+
 static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
 {
-       unsigned long int max_sz = PAGE_SIZE - sizeof(struct padded_vnet_hdr);
        struct virtnet_info *vi = netdev_priv(dev);
        struct bpf_prog *old_prog;
+       unsigned long int max_sz;
        u16 xdp_qp = 0, curr_qp;
        int i, err;
 
@@ -1720,6 +1737,7 @@ static int virtnet_xdp_set(struct net_device *dev, struct 
bpf_prog *prog)
                return -EINVAL;
        }
 
+       max_sz = virtnet_xdp_mtu(prog, vi);
        if (dev->mtu > max_sz) {
                netdev_warn(dev, "XDP requires MTU less than %lu\n", max_sz);
                return -EINVAL;
@@ -1748,6 +1766,9 @@ static int virtnet_xdp_set(struct net_device *dev, struct 
bpf_prog *prog)
                        virtnet_set_queues(vi, curr_qp);
                        return PTR_ERR(prog);
                }
+               dev->max_mtu = max_sz;
+       } else {
+               dev->max_mtu = ETH_MAX_MTU;
        }
 
        vi->xdp_queue_pairs = xdp_qp;
@@ -2133,9 +2154,6 @@ static bool virtnet_validate_features(struct 
virtio_device *vdev)
        return true;
 }
 
-#define MIN_MTU ETH_MIN_MTU
-#define MAX_MTU ETH_MAX_MTU
-
 static int virtnet_probe(struct virtio_device *vdev)
 {
        int i, err;