Re: [dpdk-dev] [PATCH] mbuf: support dynamic fields and flags

["Wang, Haiyue" <haiyue.wang@intel.com>]

> -----Original Message-----
> From: Olivier Matz [mailto:olivier.matz@6wind.com]
> Sent: Thursday, September 19, 2019 00:55
> To: dev@dpdk.org
> Cc: Thomas Monjalon <thomas@monjalon.net>; Wang, Haiyue <haiyue.wang@intel.com>; Stephen Hemminger
> <stephen@networkplumber.org>; Andrew Rybchenko <arybchenko@solarflare.com>; Wiles, Keith
> <keith.wiles@intel.com>; Jerin Jacob Kollanukkaran <jerinj@marvell.com>
> Subject: [PATCH] mbuf: support dynamic fields and flags
> 
> Many features require to store data inside the mbuf. As the room in mbuf
> structure is limited, it is not possible to have a field for each
> feature. Also, changing fields in the mbuf structure can break the API
> or ABI.
> 
> This commit addresses these issues, by enabling the dynamic registration
> of fields or flags:
> 
> - a dynamic field is a named area in the rte_mbuf structure, with a
>   given size (>= 1 byte) and alignment constraint.
> - a dynamic flag is a named bit in the rte_mbuf structure.
> 
> The typical use case is a PMD that registers space for an offload
> feature, when the application requests to enable this feature.  As
> the space in mbuf is limited, the space should only be reserved if it
> is going to be used (i.e when the application explicitly asks for it).
> 
> The registration can be done at any moment, but it is not possible
> to unregister fields or flags for now.
> 
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> Acked-by: Thomas Monjalon <thomas@monjalon.net>
> ---
> 
> rfc -> v1
> 
> * Rebase on top of master
> * Change registration API to use a structure instead of
>   variables, getting rid of #defines (Stephen's comment)
> * Update flag registration to use a similar API as fields.
> * Change max name length from 32 to 64 (sugg. by Thomas)
> * Enhance API documentation (Haiyue's and Andrew's comments)
> * Add a debug log at registration
> * Add some words in release note
> * Did some performance tests (sugg. by Andrew):
>   On my platform, reading a dynamic field takes ~3 cycles more
>   than a static field, and ~2 cycles more for writing.
> 
>  app/test/test_mbuf.c                   | 114 ++++++-
>  doc/guides/rel_notes/release_19_11.rst |   7 +
>  lib/librte_mbuf/Makefile               |   2 +
>  lib/librte_mbuf/meson.build            |   6 +-
>  lib/librte_mbuf/rte_mbuf.h             |  25 +-
>  lib/librte_mbuf/rte_mbuf_dyn.c         | 408 +++++++++++++++++++++++++
>  lib/librte_mbuf/rte_mbuf_dyn.h         | 163 ++++++++++
>  lib/librte_mbuf/rte_mbuf_version.map   |   4 +
>  8 files changed, 724 insertions(+), 5 deletions(-)
>  create mode 100644 lib/librte_mbuf/rte_mbuf_dyn.c
>  create mode 100644 lib/librte_mbuf/rte_mbuf_dyn.h
> 

[snip]

> +/**
> + * Helper macro to access to a dynamic field.
> + */
> +#define RTE_MBUF_DYNFIELD(m, offset, type) ((type)((uintptr_t)(m) + (offset)))

How about to change it as: ?
#define RTE_MBUF_DYNFIELD(m, offset, type) ((type *)((uintptr_t)(m) + (offset)))
                                                  ^
Then,
	*RTE_MBUF_DYNFIELD(mb, xxx, uint32_t) = yyy;

Since we use 'type' like: sizeof(type), __alignof__(type), this makes 'type' be
more consistent, not have to force cast 'type *' when using it.

	const struct rte_mbuf_dynfield dynfield2 = {
		.name = "test-dynfield2",
		.size = sizeof(uint16_t),
		.align = __alignof__(uint16_t),
		.flags = 0,
	};

And also, when I'm trying to use the dynamic flag, found a macro will be better
for making code align with dynamic field. Just a small suggestion. ;-)
	mb->ol_flags |= RTE_MBUF_DYNFLAG(ol_offset);

/**
 * Helper macro to access to a dynamic flag.
 */
#define RTE_MBUF_DYNFLAG(offset) (1ULL << (offset))

> +
> +#endif
> diff --git a/lib/librte_mbuf/rte_mbuf_version.map b/lib/librte_mbuf/rte_mbuf_version.map
> index 2662a37bf..a98310570 100644
> --- a/lib/librte_mbuf/rte_mbuf_version.map
> +++ b/lib/librte_mbuf/rte_mbuf_version.map
> @@ -50,4 +50,8 @@ EXPERIMENTAL {
>  	global:
> 
>  	rte_mbuf_check;
> +	rte_mbuf_dynfield_lookup;
> +	rte_mbuf_dynfield_register;
> +	rte_mbuf_dynflag_lookup;
> +	rte_mbuf_dynflag_register;
>  } DPDK_18.08;
> --
> 2.20.1