[RFC v2] eal: increase passed max multi-process file descriptors

[Stephen Hemminger]

On Fri,  8 Mar 2024 12:36:39 -0800
Stephen Hemminger <stephen at networkplumber.org> wrote:

> Both XDP and TAP device are limited in the number of queues
> because of limitations on the number of file descriptors that
> are allowed. The original choice of 8 was too low; the allowed
> maximum is 253 according to unix(7) man page.
> 
> This may look like a serious ABI breakage but it is not.
> It is simpler for everyone if the limit is increased rather than
> building a parallel set of calls.
> 
> The case that matters is older application registering MP support
> with the newer version of EAL. In this case, since the old application
> will always send the more compact structure (less possible fd's)
> it is OK.
> 
> Request (for up to 8 fds) sent to EAL.
>    - EAL only references up to num_fds.
>    - The area past the old fd array is not accessed.
> 
> Reply callback:
>    - EAL will pass pointer to the new (larger structure),
>      the old callback will only look at the first part of
>      the fd array (num_fds <= 8).
> 
>    - Since primary and secondary must both be from same DPDK version
>      there is normal way that a reply with more fd's could be possible.
>      The only case is the same as above, where application requested
>      something that would break in old version and now succeeds.
> 
> The one possible incompatibility is that if application passed
> a larger number of fd's (32?) and expected an error. Now it will
> succeed and get passed through.
> 
> Fixes: bacaa2754017 ("eal: add channel for multi-process communication")
> Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> ---
> v2 - show the simpler way to address with some minor ABI issue
> 
>  doc/guides/rel_notes/release_24_03.rst | 4 ++++
>  lib/eal/include/rte_eal.h              | 2 +-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/guides/rel_notes/release_24_03.rst b/doc/guides/rel_notes/release_24_03.rst
> index 932688ca4d82..1d33cfa15dfb 100644
> --- a/doc/guides/rel_notes/release_24_03.rst
> +++ b/doc/guides/rel_notes/release_24_03.rst
> @@ -225,6 +225,10 @@ API Changes
>  * ethdev: Renamed structure ``rte_flow_action_modify_data`` to be
>    ``rte_flow_field_data`` for more generic usage.
>  
> +* eal: The maximum number of file descriptors allowed to be passed in
> +  multi-process requests is increased from 8 to the maximum possible on
> +  Linux unix domain sockets 253. This allows for more queues on XDP and
> +  TAP device.
>  
>  ABI Changes
>  -----------
> diff --git a/lib/eal/include/rte_eal.h b/lib/eal/include/rte_eal.h
> index c2256f832e51..cd84fcdd1bdb 100644
> --- a/lib/eal/include/rte_eal.h
> +++ b/lib/eal/include/rte_eal.h
> @@ -155,7 +155,7 @@ int rte_eal_primary_proc_alive(const char *config_file_path);
>   */
>  bool rte_mp_disable(void);
>  
> -#define RTE_MP_MAX_FD_NUM	8    /* The max amount of fds */
> +#define RTE_MP_MAX_FD_NUM	253    /* The max amount of fds */
>  #define RTE_MP_MAX_NAME_LEN	64   /* The max length of action name */
>  #define RTE_MP_MAX_PARAM_LEN	256  /* The max length of param */
>  struct rte_mp_msg {


Rather than mess with versioning everything, probably better to just
hold off to 24.11 release and do the change there.

It will limit xdp and tap PMD's to 8 queues but no user has been
demanding more yet.