Re: [PATCH v4 1/4] eal: add macro to embed information in binaries

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Apr 16, 2025 at 02:44:55PM -0700, Andre Muezerie wrote:
> DPDK uses GCC attribute "used" through macro __rte_used to indicate
> that a variable not referenced in the code should be assumed being
> used and therefore not be optimized away. This technique is used to embed
> information in the binaries, by having crafted information stored in
> them.
> 
> MSVC offers similar functionality, but it differs significantly: MSVC
> requires a pragma to be used to send a command to the linker telling it
> explicitly the name of the symbol that should be included (even if not
> referenced). As a side-effect, variables called out to be included cannot
> be static, otherwise their symbols are not "seen" by the linker. This
> restriction requires some DPDK code to be refactored.
> 
> To assimilate these requirements/restrictions, macro RTE_INCLUDE is
> introduced in this patch.
> 
> Signed-off-by: Andre Muezerie <andremue@linux.microsoft.com>
> ---
>  lib/eal/common/eal_common_options.c | 2 +-
>  lib/eal/include/rte_common.h        | 6 ++++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/eal/common/eal_common_options.c b/lib/eal/common/eal_common_options.c
> index b6fff7ec05..8d115a5bd7 100644
> --- a/lib/eal/common/eal_common_options.c
> +++ b/lib/eal/common/eal_common_options.c
> @@ -135,7 +135,7 @@ static const char *default_solib_dir = RTE_EAL_PMD_PATH;
>   * Note: PLEASE DO NOT ALTER THIS without making a corresponding
>   * change to usertools/dpdk-pmdinfo.py
>   */
> -static const char dpdk_solib_path[] __rte_used =
> +RTE_INCLUDE(const char, dpdk_solib_path)[] =
>  "DPDK_PLUGIN_PATH=" RTE_EAL_PMD_PATH;
>  
>  TAILQ_HEAD(device_option_list, device_option);
> diff --git a/lib/eal/include/rte_common.h b/lib/eal/include/rte_common.h
> index a6085dce27..4c5a3b668f 100644
> --- a/lib/eal/include/rte_common.h
> +++ b/lib/eal/include/rte_common.h
> @@ -231,6 +231,12 @@ typedef uint16_t unaligned_uint16_t;
>  /**
>   * Force symbol to be generated even if it appears to be unused.
>   */
> +#ifdef RTE_TOOLCHAIN_MSVC
> +#define RTE_INCLUDE(type, name) __pragma(comment(linker, "/include:" RTE_STR(name))) type name
> +#else
> +#define RTE_INCLUDE(type, name) __attribute__((used)) type name

Any reason we shouldn't also have "static" here, for the non-MSVC case?

As a general question, is this needed for any/many tasks other than putting
in place strings? Rather than having a general include macro,
I'm just wondering if we are better doing specific macros for the driver
string, and then other-string use-cases.

/Bruce