[...] > On 2017年01月04日 00:48, John Fastabend wrote: >> On 17-01-02 10:14 PM, Jason Wang wrote: >>> >>> On 2017年01月03日 06:30, John Fastabend wrote: >>>> 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> >>>> ---
[...] >> 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. > > Yes, this is a problem (even use ETH_MAX_MTU). We may need a method to notify > the device about the mtu in this case which is not supported by virtio now. Note this is not really a XDP specific problem. The guest can change the MTU after init time even without XDP which I assume should ideally result in a notification if the MTU is negotiated. >> >> 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(). > > This seems something misunderstanding here, I meant only use GOOD_PACKET_LEN > for > small buffer (which does not use EWMA). > Yep I think its all cleared up now. Thanks.
