On 6/9/25 23:58, Jakub Kicinski wrote:

On Fri, 6 Jun 2025 22:48:53 +0700 Bui Quang Minh wrote:

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But currently, if a multi-buffer packet arrives, it will not go through
XDP program so it doesn't increase the stats but still goes to network
stack. So I think it's not a correct behavior.

Sounds fair, but at a glance the normal XDP path seems to be trying to
linearize the frame. Can we not try to flatten the frame here?
If it's simply to long for the chunk size that's a frame length error,
right?

Here we are in the zerocopy path, so the buffers for the frame to fill
in are allocated from XDP socket's umem. And if the frame spans across
multiple buffers then the total frame size is larger than the chunk
size.

Is that always the case? Can the multi-buf not be due to header-data
split of the incoming frame? (I'm not familiar with the virtio spec)

Ah, maybe I cause some confusion :) zerocopy here means zerocopy if the 
frame is redirected to XDP socket. In this zerocopy mode, XDP socket 
will provide buffers to virtio-net, the frame from vhost will be placed 
in those buffers. If the bind XDP program in virtio-net returns 
XDP_REDIRECT to that XDP socket, then the frame is zerocopy. In case 
XDP_PASS is returned, the frame's data is copied to newly created skb 
and the frame's buffer is returned to XDP socket. AFAIK, virtio-net has 
not supported header-data split yet.

quoted

Furthermore, we are in the zerocopy so we cannot linearize by
allocating a large enough buffer to cover the whole frame then copy the
frame data to it. That's not zerocopy anymore. Also, XDP socket zerocopy
receive has assumption that the packet it receives must from the umem
pool. AFAIK, the generic XDP path is for copy mode only.

Generic XDP == do_xdp_generic(), here I think you mean the normal XDP
patch in the virtio driver? If so then no, XDP is very much not
expected to copy each frame before processing.

Yes, I mean generic XDP = do_xdp_generic(). I mean that we can linearize 
the frame if needed (like in netif_skb_check_for_xdp()) in copy mode for 
XDP socket but not in zerocopy mode.

This is only slightly related to you patch but while we talk about
multi-buf - in the netdev CI the test which sends ping while XDP
multi-buf program is attached is really flaky :(
https://netdev.bots.linux.dev/contest.html?executor=vmksft-drv-hw&test=ping-py.ping-test-xdp-native-mb&ld-cases=1

metal-drv-hw means the NETIF is the real NIC, right?

Thanks,
Quang Minh.