meson option to customize RTE_PKTMBUF_HEADROOM patch

[Garrett D'Amore]

So we right now (at WEKA) have a somewhat older version of DPDK that we have customized heavily, and I am going to to need to to make the headroom *dynamic* (passed in at run time, and per port.)

We have this requirement because we need payload to be at a specific offset, but have to deal with different header lengths for IPv4 and now IPv6.

My reason for pointing this out, is that I would dearly like if we could collaborate on this -- this change is going to touch pretty much every PMD (we don't need it on all of them as we only support a subset of PMDs, but its still a significant set.)

I'm not sure if anyone else has considered such a need -- this particular message caught my eye as I'm looking specifically in this area right now.
On Feb 15, 2024 at 11:02 AM -0800, Parthakumar Roy <Parthakumar.Roy at ibm.com>, wrote:
> Hello,
> Bruce Richardson suggested that I submit this patch - at IBM we needed to adjust the pkt_mbuf_headroom​ for our application to work. This is my first ever patch through a mailing list, I have only done it through Pull Requests before, so let me know if I need to correct something.
>
> Message:
>     Add meson configuration option to adjust RTE_PKTMBUF_HEADROOM
>
> diff --git a/config/meson.build b/config/meson.build
> index 7cd375e991..43b765ade1 100644
> --- a/config/meson.build
> +++ b/config/meson.build
> @@ -304,6 +304,7 @@ endforeach
>  dpdk_conf.set('RTE_MAX_ETHPORTS', get_option('max_ethports'))
>  dpdk_conf.set('RTE_LIBEAL_USE_HPET', get_option('use_hpet'))
>  dpdk_conf.set('RTE_ENABLE_TRACE_FP', get_option('enable_trace_fp'))
> +dpdk_conf.set('RTE_PKTMBUF_HEADROOM', get_option('pkt_mbuf_headroom'))
>  # values which have defaults which may be overridden
>  dpdk_conf.set('RTE_MAX_VFIO_GROUPS', 64)
>  dpdk_conf.set('RTE_DRIVER_MEMPOOL_BUCKET_SIZE_KB', 64)
> diff --git a/config/rte_config.h b/config/rte_config.h
> index 7b8c85e948..a2bb4ea61b 100644
> --- a/config/rte_config.h
> +++ b/config/rte_config.h
> @@ -51,7 +51,6 @@
>
>  /* mbuf defines */
>  #define RTE_MBUF_DEFAULT_MEMPOOL_OPS "ring_mp_mc"
> -#define RTE_PKTMBUF_HEADROOM 128
>
>  /* ether defines */
>  #define RTE_MAX_QUEUES_PER_PORT 1024
> diff --git a/meson_options.txt b/meson_options.txt
> index 08528492f7..169fcc94c7 100644
> --- a/meson_options.txt
> +++ b/meson_options.txt
> @@ -36,6 +36,8 @@ option('machine', type: 'string', value: 'auto', description:
>         'Alias of cpu_instruction_set.')
>  option('max_ethports', type: 'integer', value: 32, description:
>         'maximum number of Ethernet devices')
> +option('pkt_mbuf_headroom', type: 'integer', value: 128, description:
> +       'number of bytes skipped on Rx at the start of the packet buffer to leave room for additional packet headers')
>  option('max_lcores', type: 'string', value: 'default', description:
>         'Set maximum number of cores/threads supported by EAL; "default" is different per-arch, "detect" detects the number of cores on the build machine.')
>  option('max_numa_nodes', type: 'string', value: 'default', description:
>
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