Re: [PATCH v3 net-next 02/15] selftests: xsk: size the TX UMEM in HW_SW_MAX_RING_SIZE

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Sat, Oct 17, 2026 at 03:06:03AM +0000, netdev-bot+sashiko@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential
> issue(s) to consider.
> 
> Critical: 0 · High: 0 · Medium: 1 · Low: 0
> 
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> 
> - [Medium] In testapp_hw_sw_max_ring_size()
>   (tools/testing/selftests/bpf/prog_tests/test_xsk.c), the patch gives…
> 
> --
> 
> selftests: xsk: size the TX UMEM in HW_SW_MAX_RING_SIZE
> 
> In testapp_hw_sw_max_ring_size(), this patch gives the TX UMEM the same
> 8k frames, fill ring and completion ring as the RX UMEM. In two-interface
> mode the TX UMEM has only 1k frames, so the 2k batch stalls in
> __send_pkts(). The patch prepares for removing the single-interface mode
> in the next patch.
> 
> > diff --git a/tools/testing/selftests/bpf/prog_tests/test_xsk.c b/tools/testing/selftests/bpf/prog_tests/test_xsk.c
> > index 2a13628740924..16fe32e5afc06 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/test_xsk.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/test_xsk.c
> > @@ -2513,6 +2513,9 @@ int testapp_hw_sw_max_ring_size(struct test_spec *test)
> >  	test->ifobj_rx->xsk->umem->num_frames = max_descs;
> >  	test->ifobj_rx->xsk->umem->fill_size = max_descs;
> >  	test->ifobj_rx->xsk->umem->comp_size = max_descs;
> > +	test->ifobj_tx->xsk->umem->num_frames = max_descs;
> > +	test->ifobj_tx->xsk->umem->fill_size = max_descs;
> > +	test->ifobj_tx->xsk->umem->comp_size = max_descs;
> 
> [Severity: Medium]
> Does the second step still stall on NICs that report a tx_max_pending of
> 8200 or more?
> 
> max_descs is fixed at 4 * XSK_RING_PROD__DEFAULT_NUM_DESCS, which is 8192.
> The step-2 batch later in testapp_hw_sw_max_ring_size() still comes from
> the maximum the driver reports:
> 
> 	test->ifobj_tx->xsk->batch_size = test->ifobj_tx->ring.tx_max_pending - 8;
> 
> Here is the in-flight limit check in __send_pkts():
> 
> 	if (pkts_in_flight >= (int)((umem_size(umem) - xsk->batch_size * buffer_len) /
> 	    buffer_len) && !test_timeout) {
> 
> With batch_size == 8192 the limit is 0. With batch_size > 8192 the u64
> subtraction wraps, and the truncated int is 8192 - batch_size, which is
> negative. In both cases TX only kicks and returns TEST_CONTINUE.
> 
> Even if that check passed, xsk_ring_prod__reserve(&xsk->tx, batch_size,
> ...) can never reserve more than the 8192 slots of the TX ring. This is
> because xsk_configure_socket() sets cfg.tx_size = umem->comp_size.
> 
> Some hardware that supports AF_XDP reports larger maximums:
> 
> - octeontx2: otx2_get_ringparam() reports tx_max_pending =
>   Q_COUNT(Q_SIZE_MAX), which is 1048576. The batch is 1048568 and the
>   limit is -1040376.
> - hns3: reports HNS3_RING_MAX_PENDING, which is 32760. The batch is 32752
>   and the limit is -24560.
> 
> ice (8160) and mlx5 (8192) fit within the limit.
> 
> On the larger NICs, TX never sends and RX times out. That seems to be the
> same failure the commit message describes:
> 
>   "the in-flight limit in __send_pkts() turns negative for a 2k batch,
>   so TX never sends and RX times out, and the second batch would not fit
>   in the TX ring anyway."
> 
> At the end of the series, tools/testing/selftests/net/lib/xsk/test_xsk.c
> still uses the fixed 8192 and xsk_get_cap(test, tx_max_pending) - 8. Once
> the single-interface mode is gone, two-interface mode is the only one
> left, so this case would keep failing on those NICs.
> 
> A 1M-frame UMEM would need 4 GiB. Could the step-2 batch be clamped to
> the software ring instead, for example min(tx_max_pending, max_descs) - 8?

I have sent v4 already which addresses this:

	batch_size = min(tx_max_pending, max_descs) - 8;
	test->ifobj_tx->xsk->batch_size = batch_size;
	test->ifobj_rx->xsk->batch_size = batch_size;

> 
> [ ... ]
> 
> -- 
> Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261014184539.774708-1-maciej.fijalkowski%40intel.com