Re: [PATCH v7 0/4] net/af_packet: do not return outgoing packets from rte_eth_rx_burst

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 16 Oct 2026 22:19:38 +0100
Frank Dressler <frank@dressler.pro> wrote:

> The third patch adds PACKET_IGNORE_OUTGOING which moves the
> filtering into the kernel. It is supported since Linux 4.20 but only
> works for single-queue AF_PACKET scenarios. Multi-queue AF_PACKET
> is still filtered in the PMD.
> 
> The fourth patch adds PACKET_FANOUT_FLAG_IGNORE_OUTGOING which is
> supported since Linux 6.2 and moves the filtering into the kernel
> for the multi-queue scenarios. The kernel does not expose a direct
> way to check whether it supports PACKET_FANOUT_FLAG_IGNORE_OUTGOING
> but it also does not return an error when the sockopt is set on
> kernels that do not support it. Because of that the sockopt is set
> opportunistically. The PMD check is kept, since older kernels accept
> the flag without effect and support cannot be detected.
> 
> Overall we get the following:
> 
>               | Single Queue | Multi-Queue
>   ------------+--------------+-------------
>   Kernel 5.4+ |  in kernel   | in PMD
>   Kernel 6.2+ |  in kernel   | in kernel
> 
> I tested this on both, kernel 7.2.8 and kernel 5.4.0 with both single
> and multiple queues. My results, strace'ing testpmd 7.2.8, with the
> variants of ignore_outgoing=0|1 and single/multi-queue:
> 
> Kernel 5.4.0:
> 	qpairs=1,ignore_outgoing=0: -
> 	qpairs=1,ignore_outgoing=1: setsockopt(14, SOL_PACKET, 0x17 /* PACKET_??? */, [1], 4) = 0
> 	// 0x17 is PACKET_IGNORE_OUTGOING
> 
> 	qpairs=2,ignore_outgoing=0: setsockopt(14, SOL_PACKET, PACKET_FANOUT, [-1878917532], 4) = 0
> 	// -1878917532 does NOT contain PACKET_FANOUT_FLAG_IGNORE_OUTGOING<<16
> 	qpairs=2,ignore_outgoing=1: setsockopt(14, SOL_PACKET, PACKET_FANOUT, [-805175699], 4) = 0
> 	// -805175699 does contain PACKET_FANOUT_FLAG_IGNORE_OUTGOING<<16
> 
> Kernel 7.2.8:
> 	qpairs=1,ignore_outgoing=0: -
> 	qpairs=1,ignore_outgoing=1: setsockopt(13, SOL_PACKET, PACKET_IGNORE_OUTGOING, [1], 4) = 0
> 
> 	qpairs=2,ignore_outgoing=0: setsockopt(13, SOL_PACKET, PACKET_FANOUT, [-1878978899], 4) = 0
> 	// -1878978899 does NOT contain PACKET_FANOUT_FLAG_IGNORE_OUTGOING<<16
> 	qpairs=2,ignore_outgoing=1: setsockopt(13, SOL_PACKET, PACKET_FANOUT, [-805236964], 4) = 0
> 	// -805236964 does contain PACKET_FANOUT_FLAG_IGNORE_OUTGOING<<16
> 
> They show that the setsockopt succeeds even on the old kernel that
> does not support PACKET_FANOUT_FLAG_IGNORE_OUTGOING, so setting it
> opportunistically is fine. They also show that the code only tries to
> set PACKET_FANOUT_FLAG_IGNORE_OUTGOING or PACKET_IGNORE_OUTGOING when
> requested by ignore_outgoing=1. And it shows that
> PACKET_IGNORE_OUTGOING is used for single and
> PACKET_FANOUT_FLAG_IGNORE_OUTGOING for multi-queue.
> 
> I have also manually ran testpmd in all modes and both kernels to
> verify that ignore_outgoing works as expected, sending packets and
> checking what testpmd was receiving (``set verbose 1``).
> 
> 
> v7:
> - log ignore_outgoing with %u
> - use n_inject for both sides of the ignore_outgoing test
> - mention Linux 4.20 once in the single-queue commit message
> - keep the release note to the devarg, not where filtering happens

Applied to next-net with simplification of the release note.