RFC acceptable handling of VLAs across toolchains

[Morten Brørup]

> From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> Sent: Wednesday, 8 November 2023 17.52
> 
> On Tue, 7 Nov 2023 11:32:20 -0800
> Tyler Retzlaff <roretzla at linux.microsoft.com> wrote:
> 
> > hi folks,
> >
> > i'm seeking advice. we have use of VLAs which are now optional in
> > standard C. some toolchains provide a conformant implementation and
> msvc
> > does not (and never will).
> >
> > we seem to have a few options, just curious about what people would
> > prefer.
> >
> > * use alloca
> >
> > * use dynamically allocated storage
> >
> > * conditional compiled code where the msvc leg uses one of the
> previous
> >   two options
> >
> > i'll leave it simple for now, i'd like to hear input rather than
> provide
> > a recommendation for now.
> >
> > thanks!
> 
> As an experiment did a build of current DPDK with -Wvla option.
> 
> Lots of errors, some have obvious solutions like:
> 
> ../drivers/net/failsafe/failsafe_intr.c: In function
> ‘fs_rx_event_proxy_service_install’:
> ../drivers/net/failsafe/failsafe_intr.c:142:17: warning: ISO C90
> forbids variable length array ‘service_core_list’ [-Wvla]
>   142 |                 uint32_t service_core_list[num_service_cores];
>       |                 ^~~~~~~~
> 
> This could just be RTE_MAX_LCORES.
> 
> others like rte_metrics should just use malloc() as is used already in
> that function.
> 
> ../lib/metrics/rte_metrics_telemetry.c: In function
> ‘rte_metrics_tel_update_metrics_ethdev’:
> ../lib/metrics/rte_metrics_telemetry.c:140:9: warning: ISO C90 forbids
> variable length array ‘xstats_values’ [-Wvla]
>   140 |         uint64_t xstats_values[num_xstats];
>       |         ^~~~~~~~
> ../lib/metrics/rte_metrics_telemetry.c: In function
> ‘rte_metrics_tel_extract_data’:
> ../lib/metrics/rte_metrics_telemetry.c:384:9: warning: ISO C90 forbids
> variable length array ‘stat_names’ [-Wvla]
>   384 |         const char *stat_names[num_stat_names];
> |         ^~~~~
> 
> Others already have an implicit upper bound.
> Example is in rte_cuckoo_hash where some fields us
> RTE_HASH_LOOKUP_BULK_MAX
> and some use VLA.
> 
> [170/2868] Compiling C object
> lib/librte_hash.a.p/hash_rte_cuckoo_hash.c.o
> ../lib/hash/rte_cuckoo_hash.c: In function ‘rte_hash_lookup_bulk_data’:
> ../lib/hash/rte_cuckoo_hash.c:2355:9: warning: ISO C90 forbids variable
> length array ‘positions’ [-Wvla]
>  2355 |         int32_t positions[num_keys];
>       |         ^~~~~~~
> ../lib/hash/rte_cuckoo_hash.c: In function
> ‘rte_hash_lookup_with_hash_bulk_data’:
> ../lib/hash/rte_cuckoo_hash.c:2471:9: warning: ISO C90 forbids variable
> length array ‘positions’ [-Wvla]
>  2471 |         int32_t positions[num_keys];
>       |         ^~~~~~~
> 
> Would it make sense to have an rte_config.h value for maximum burst
> size?

I would support that! There could be a few burst size defines, e.g.

- SMALL: used for small bursts (I think some drivers use bursts of 8)
- NORMAL: used for typical bursts
- LARGE: used for large bursts, e.g. mempool cache flush

Having these available at build time would also allow more optimizations in DPDK libs and drivers for those specific burst sizes.


> Lots of code is using nb_pkts.
> 
> There is also some confusing ones like:
> ../lib/mempool/rte_mempool.c: In function ‘mempool_cache_init’:
> ../lib/mempool/rte_mempool.c:751:50: warning: ISO C90 forbids array
> whose size cannot be evaluated [-Wvla]
>   751 |                          RTE_SIZEOF_FIELD(struct
> rte_mempool_cache, objs[0]));
>       |
> ^~~~~~~~~~~~~~~~~
> ../lib/eal/include/rte_common.h:498:65: note: in definition of macro
> ‘RTE_BUILD_BUG_ON’
>   498 | #define RTE_BUILD_BUG_ON(condition) ((void)sizeof(char[1 -
> 2*!!(condition)]))
>       |
> ^~~~~~~~~
> ../lib/mempool/rte_mempool.c:751:26: note: in expansion of macro
> ‘RTE_SIZEOF_FIELD’
>   751 |                          RTE_SIZEOF_FIELD(struct
> rte_mempool_cache, objs[0]));