Re: [net PATCH] net: virtio: cap mtu when XDP programs are running

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 <redacted>
---
  drivers/net/virtio_net.c |    9 ++++++---
  1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 5deeda6..783e842 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -1699,6 +1699,9 @@ 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 int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
  {
  	unsigned long int max_sz = PAGE_SIZE - sizeof(struct padded_vnet_hdr);

@@ -1748,6 +1751,9 @@ 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 +2139,6 @@ 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;