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

[Olivier Matz <olivier.matz@6wind.com>]

Hi,

On Sat, Sep 21, 2019 at 04:54:39AM +0000, Wang, Haiyue wrote:
> > -----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,
> 	};

Yes, I don't see use cases where the '*' is omitted, so it could be in the
macro. On the other hand, doing like in the patch is more consistent with
similar macros like rte_pktmbuf_mtod(), so I'll tend to keep it as is.

This is maybe not that important, because this macro will often be hidden
in a wrapper, like below:

  static inline uint64_t rte_mbuf_dyn_timestamp_get(const struct rte_mbuf *m)
  {
         return *RTE_MBUF_DYNFIELD(m, rte_mbuf_dynfield_timestamp_offset,
                                 uint64_t *);
  }


> 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))

OK, I will add it in next version.



Thank you for the feedback!

Olivier