Re: [PATCH net-next v2 1/2] selftests: drv-net: devmem: fix 5-tuple flow steering

On Thu, Aug 6, 2026 at 12:16 PM Jakub Kicinski <[email protected]> wrote:
>
> On Mon,  3 Aug 2026 18:18:25 +0000 Mina Almasry wrote:
> > When configuring traffic for the devmem tests, 5-tuple flow steering may
> > be required if the environment does not provide network namespaces.
> > Pass the correct remote address parameter and fall back to 5-tuple flow
> > steering if netns is not available.
>
> Could you clarify what existing upstream test / scenario needs this?

This is needed to give the check_rx_hds test a chance to pass on
5-tuple flow steering driver, like GVE. Without this change:

    # Exception| Traceback (most recent call last):
    # Exception|   File
"/usr/src/kernel/tools/testing/selftests/drivers/net/hw/devmem_lib.py",
line 219, in run_rx_hds
    # Exception|     wait_port_listen(port, proto="tcp", ns=netns)
    ...
    # Exception| net.lib.py.utils.CmdExitFailure: Command failed
    # Exception| CMD:
/usr/src/kernel/tools/testing/selftests/drivers/net/hw/ncdevmem -l -f
eth1 -s 192.168.1.84 -p 46391 -L
    # Exception|   EXIT: 1
    # Exception|   STDERR: using ifindex=3
    # Exception|           using queues 15..16
    # Exception|           TCP header split: on
    # Exception|           Running: ethtool -X eth1 equal 15 >&2
    # Exception|           Running: ethtool -N eth1 flow-type tcp4
dst-ip 192.168.1.84   dst-port 46391 queue 15
    # Exception|           rmgr: Cannot insert RX class rule: Invalid argument
    # Exception|           Cannot insert classification rule
    # Exception|           Running: ethtool -N eth1 flow-type tcp4
dst-ip 192.168.1.84 dst-port 46391 queue 15
    # Exception|           rmgr: Cannot insert RX class rule: Invalid
argument
    # Exception|           Cannot insert classification rule
    # Exception|           ncdevmem: Failed to configure flow steering
    # Exception|           Running: ethtool -X eth1 default >&2
    # Exception|
    not ok 4 devmem.check_rx_hds

With this change:

    ok 1 devmem.check_rx
    ok 2 devmem.check_tx
    ok 3 devmem.check_tx_chunks
    # Exception| Traceback (most recent call last):
    # Exception|   File
"/usr/src/kernel/tools/testing/selftests/net/lib/py/ksft.py", line
420, in ksft_run
    # Exception|     func(*args)
    # Exception|   File
"/usr/src/kernel/tools/testing/selftests/drivers/net/hw/devmem.py",
line 30, in check_rx_hds
    # Exception|     run_rx_hds(cfg)
    # Exception|   File
"/usr/src/kernel/tools/testing/selftests/drivers/net/hw/devmem_lib.py",
line 219, in run_rx_hds
    # Exception|     wait_port_listen(port, proto="tcp", ns=netns)
    # Exception|   File
"/usr/src/kernel/tools/testing/selftests/net/lib/py/utils.py", line
356, in wait_port_listen
    # Exception|     raise Exception("Waiting for port listen timed out")
    # Exception| Exception: Waiting for port listen timed out
    # Exception|
    not ok 4 devmem.check_rx_hds

Yes the test fails, but it gets past the flow steering configuration problem.

And in the code, the change looks correct to me. If flow_steer is
False, then we don't pass the -c arg to ncdevmem. ncdevmem is written
so that if -c is not set, 5-tuple flow steering is not possible (the
client side port is random), so it falls back to 3-tuple flow
steering, which doesn't work on GVE or any other driver that supports
only 5-tuple flow steering.

run_rx (devmem.check_rx test case) doesn't have this problem because
it does `flow_steer=not hasattr(cfg, 'netns')`, but run_rx_hds has
this problem because it forgets to override flow_steer at all. This
patch fixes that.

To be honest while looking at this there were a few weird things. Like
I'm not sure why the python arg is named `flow_steer` (it likely
should be 5_tuple_flow_steer), and why flow_steer setting is tied to
hasattr(cfg, 'netns'), it should be a proprety of the driver you're
runing on (unless all in-netns drivers will support 3-tuple flow
steering).

I am planning to debug the remaining failure running check_rx_hds on
GVE and following up with another patch for that.

-- 
Thanks,
Mina