Re: Driver has suspect GRO implementation, TCP performance may be compromised.

On 5/30/19 3:52 PM, Alexander Duyck wrote:

Actually I think there are some parts that don't have any receive
limits that are supported by the e1000 part. What ends up happening is
that we only drop the packet if it spans more than one buffer if I
recall correctly, and buffer size is determined by MTU.

I always thought MTU only applied to transmit since it is kind of in
the name. As a result I am pretty sure igb and ixgbe will be able to
trigger this warning under certain circumstances as well. Also what
about the case where someone sets the MTU to less than 1500? I think
most NICs probably don't update their limits in such a case and
wouldn't it also trigger a similar error?

Linux does not have a notion of MRU, mtu is used for both tx and rx.

Most NIC drivers allocate skb of the maximal size (derived from netdev->mtu)
and program the NIC to drop packets bigger than X bytes  (X also derived from netdev->mtu)

Another interesting point is that Paul host is receiving big packets,
that means that one host in his local network is overriding the 1500 MTU :)

Eventually we could add a netdev->mru and allow few drivers to set
their maximal mru, if bigger than netdev->mtu.

e1000e would probably set netdev->mru to 2048 - sizeof(ethernet headers), if
the driver is operating at MTU = 1500