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From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Stephen Hemminger" <stephen@networkplumber.org>,
	"Tyler Retzlaff" <roretzla@linux.microsoft.com>
Cc: <dev@dpdk.org>
Subject: RE: RFC acceptable handling of VLAs across toolchains
Date: Wed, 8 Nov 2023 18:48:56 +0100	[thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9EFF7@smartserver.smartshare.dk> (raw)
In-Reply-To: <20231108085154.757719e4@hermes.local>

> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Wednesday, 8 November 2023 17.52
> 
> On Tue, 7 Nov 2023 11:32:20 -0800
> Tyler Retzlaff <roretzla@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]));

  reply	other threads:[~2023-11-08 17:48 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-11-07 19:32 Tyler Retzlaff
2023-11-08  2:31 ` Stephen Hemminger
2023-11-08  3:25   ` Tyler Retzlaff
2023-11-08  8:19     ` Morten Brørup
2023-11-08 16:51 ` Stephen Hemminger
2023-11-08 17:48   ` Morten Brørup [this message]
2023-11-09 10:25     ` RFC: default burst sizes in rte_config Morten Brørup
2023-11-09 20:26   ` RFC acceptable handling of VLAs across toolchains Tyler Retzlaff
2024-03-21  0:12     ` Tyler Retzlaff
2024-04-04 17:15 ` [PATCH 0/4] RFC samples converting VLA to alloca Tyler Retzlaff
2024-04-04 17:15   ` [PATCH 1/4] latencystats: use alloca instead of vla trivial Tyler Retzlaff
2024-04-06 15:28     ` Morten Brørup
2024-04-07  9:36       ` Mattias Rönnblom
2024-04-07 17:00         ` Stephen Hemminger
2024-04-04 17:15   ` [PATCH 2/4] hash: " Tyler Retzlaff
2024-04-06 16:01     ` Morten Brørup
2024-04-04 17:15   ` [PATCH 3/4] vhost: use alloca instead of vla sizeof Tyler Retzlaff
2024-04-06 22:30     ` Morten Brørup
2024-04-04 17:15   ` [PATCH 4/4] dispatcher: use alloca instead of vla multi dimensional Tyler Retzlaff
2024-04-06 15:49     ` Morten Brørup
2024-04-07  9:31   ` [PATCH 0/4] RFC samples converting VLA to alloca Mattias Rönnblom
2024-04-07 11:07     ` Morten Brørup
2024-04-07 17:03       ` Stephen Hemminger
2024-04-08 15:27         ` Tyler Retzlaff
2024-04-08 15:53           ` Morten Brørup
2024-04-09  8:28             ` Konstantin Ananyev
2024-04-09 15:08               ` Tyler Retzlaff
2024-04-10  9:58                 ` Konstantin Ananyev
2024-04-10 17:03                   ` Tyler Retzlaff
2024-04-10  7:32             ` Mattias Rönnblom
2024-04-10  7:52               ` Morten Brørup
2024-04-10 17:04               ` Tyler Retzlaff
2024-04-10  7:27           ` Mattias Rönnblom
2024-04-10 17:10             ` Tyler Retzlaff

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