Re: [PATCH net v5 2/2] selftests: net: add a vhost-net test for the tun NAPI queue bound

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Antonio Quartulli wrote:
> On 13/11/2026 00:34, Willem de Bruijn wrote:
> > Antonio Quartulli wrote:
> >> Send 16k packets from vhost-net into a tap device with IFF_NAPI and
> >> netdev_max_backlog set to 8, in the three ways vhost-net passes packets
> >> to tun: batches of XDP buffers, one packet per sendmsg() with MSG_MORE,
> >> and zerocopy buffers. Check that excess packets are dropped and counted,
> >> that the queue keeps draining while vhost-net keeps sending, that every
> >> packet is either delivered or dropped, and that every buffer comes back
> >> to the ring. If a dropped zerocopy buffer is never completed, the test
> >> fails, and closing vhost-net then blocks until the buffer is released.
> >>
> >> The zerocopy variant needs vhost_net loaded with experimental_zcopytx
> >> set and is skipped otherwise; with it, vhost-net does not batch and the
> >> batch variant is skipped instead.
> > 
> > That means it will be skipped as part of kselftests.
> > 
> > And as a misc device all tests may be skipped in minimal environments
> > if vhost-net is not autoloaded.
> > 
> > Maybe a .sh wrapper could resolve both.
> 
> Right. With a wrapper script we should be able to deal with these.
> 
> >   
> >> The test drives a minimal split virtqueue itself, in little-endian as
> >> VIRTIO_F_VERSION_1 requires. It needs root and CONFIG_VHOST_NET.
> >> netdev_max_backlog is global: the test lowers it only once setup has
> >> succeeded and restores it first in teardown, and should not run in
> >> parallel with other tests.
> >>
> >> Written and tested with the help of an AI coding assistant (Claude
> >> Code).
> >>
> >> Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
> >> ---
> >>   tools/testing/selftests/net/.gitignore  |   1 +
> >>   tools/testing/selftests/net/Makefile    |   1 +
> >>   tools/testing/selftests/net/config      |   1 +
> >>   tools/testing/selftests/net/tun_vhost.c | 409 ++++++++++++++++++++++++
> >>   4 files changed, 412 insertions(+)
> >>   create mode 100644 tools/testing/selftests/net/tun_vhost.c
> > 
> > That's a lot of code for a fairly narrow test.
> > 
> > Not that long ago I would have applauded all test coverage we can get.
> > But now that LLMs generate them instantly, not sure where to draw the
> > line.
> 
> Well, the test has already done its job because it immediately helped me 
> fix some small itches in patch 1.
> 
> This said, I can definitely shrink this test a bit, maybe around 50% of 
> the current code. But it'll still be quite some code for a small fix.
> 
> Should I just drop this patch?
> 
> Alternatively, I drop it from this series and I send the shrunk-down 
> version with the wrapper script to net-next later on.
> 
> 
> Opinions?

If you could compress it to about 50%, let's merge.

Maybe we can use it over time for more coverage in this area.