> From: Thomas Monjalon [mailto:tho...@monjalon.net]
> Sent: Thursday, 12 October 2023 01.44
>
> 04/09/2023 10:43, Morten Brørup:
> > /** Force minimum cache line alignment. */
> > #define __rte_cache_min_aligned __rte_aligned(RTE_CACHE_LINE_MIN_SIZE)
> >
> > +#define _RTE_CACHE_GUARD_HELPER2(unique) \
> > + char cache_guard_ ## unique[RTE_CACHE_LINE_SIZE *
> RTE_CACHE_GUARD_LINES] \
> > + __rte_cache_aligned
> > +#define _RTE_CACHE_GUARD_HELPER1(unique) _RTE_CACHE_GUARD_HELPER2(unique)
>
> What is the reason for this intermediate helper macro?
>
> > +/**
> > + * Empty cache lines, to guard against false sharing-like effects
> > + * on systems with a next-N-lines hardware prefetcher.
> > + *
> > + * Use as spacing between data accessed by different lcores,
> > + * to prevent cache thrashing on hardware with speculative prefetching.
> > + */
> > +#define RTE_CACHE_GUARD _RTE_CACHE_GUARD_HELPER1(__COUNTER__)
HELPER1 is required to convert __COUNTER__ to a number before HELPER2
concatenates it to cache_guard_.
If using HELPER2 directly,
#define RTE_CACHE_GUARD _RTE_CACHE_GUARD_HELPER2(__COUNTER__)
would expand to:
char cache_guard_ ## __COUNTER__ [RTE_CACHE_LINE_SIZE * RTE_CACHE_GUARD_LINES]
__rte_cache_aligned
Which is not unique, and would prevent using RTE_CACHE_GUARD multiple times in
the same structure.
