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

On 17-01-02 10:14 PM, Jason Wang wrote:

On 2017年01月03日 06:30, John Fastabend wrote:

quoted

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;

Or use ETH_DATA_LEN here consider we only allocate a size of GOOD_PACKET_LEN for
each small buffer?

Thanks

OK so this logic is a bit too simply. When it resets the max_mtu I guess it
needs to read the mtu via

   virtio_cread16(vdev, ...)

or we may break the negotiated mtu.

As for capping it at GOOD_PACKET_LEN this has the nice benefit of avoiding any
underestimates in EWMA predictions because it appears min estimates are capped
at GOOD_PACKET_LEN via get_mergeable_buf_len().

Thanks,
John