[EXTERNAL] Re: Support jumbo packets with XDP

[Stephen Hemminger]

On Mon, 3 Feb 2025 09:16:58 +0200
Ofer Dagan <ofer.d at claroty.com> wrote:

> 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
> > ZjQcmQRYFpfptBannerStart
> > ------------------------------
> > WARNING:External E-Mail - Use caution with links and attachments
> >
> > ZjQcmQRYFpfptBannerEnd
> >
> > 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

The limitation is in the kernel (not DPDK). The kernel XDP uses BPF and to optimize
with zero copies only supports up to one page. You might talk to the kernel XDP maintainers
about if they support scatter/gather.