[EXTERNAL] Re: Support jumbo packets with XDP

[Ofer Dagan]

Hi Stephan,

Thanks for your response. I'm building DPDK 24.11 on kernel version
5.15.0-130-generic. It does seem to work with the MTU you suggested, but
how can I support even larger packets (up to 9000)?
Are there any workarounds for such cases? I don't mind a performance
penalty as these use cases are expected to support less traffic then the
ones using dpdk drivers.

On Sun, Feb 2, 2025 at 7:33 PM Stephen Hemminger <stephen at networkplumber.org>
wrote:

> On Sun, 2 Feb 2025 08: 53: 42 +0200 Ofer Dagan <ofer. d@ claroty. com>
> wrote: > Hi all, > > We are trying to start using AF_XDP instead of libpcap
> (for use cases where > dpdk drivers aren't a good fit for us). When using
> XDP, we
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> On Sun, 2 Feb 2025 08:53:42 +0200
> Ofer Dagan <ofer.d at claroty.com> wrote:
>
> > Hi all,
> >
> > We are trying to start using AF_XDP instead of libpcap (for use cases where
> > dpdk drivers aren't a good fit for us). When using XDP, we can't set high
> > MTU. How to still support jumbo packets in our application?
> >
> > Thanks,
> > Ofer
>
> What error are you seeing?
> What version of DPDK, and what version of kernel are you building for.
>
> The current version of AF_XDP Poll mode driver supports larger mtu sizes.
> It is constrained because the receive buffer has to fit on a single page
> and there is overhead for the various headers.
>
>
> 	dev_info->min_mtu = RTE_ETHER_MIN_MTU;
> #if defined(XDP_UMEM_UNALIGNED_CHUNK_FLAG)
> 	dev_info->max_rx_pktlen = getpagesize() -
> 				  sizeof(struct rte_mempool_objhdr) -
> 				  sizeof(struct rte_mbuf) -
> 				  RTE_PKTMBUF_HEADROOM - XDP_PACKET_HEADROOM;
> #else
> 	dev_info->max_rx_pktlen = ETH_AF_XDP_FRAME_SIZE - XDP_PACKET_HEADROOM;
> #endif
> 	dev_info->max_mtu = dev_info->max_rx_pktlen - ETH_AF_XDP_ETH_OVERHEAD;
>
> If you have a relatively recent kernel the UNALIGNED_CHUNK_FLAG should be set.
> Stepping through the maths for that
> 	max_rx_pktlen = 4096 - 24 - 128 - 128 - 256 = 3560
> 	max_mtu = max_rx_pktlen - 14 - 4 = 3542
>
>
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