Re: [dpdk-dev] [RFC PATCH] build/pkg-config: remove machine arch flag

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

On Fri, Dec 11, 2020 at 03:51:11PM +0000, Bruce Richardson wrote:
> Traditionally any apps built using DPDK had to support the same
> instruction sets as supported when the DPDK SDK itself was built, since
> that was "leaked" through to the end-app and DPDK headers via
> RTE_MACHINE_CPUFLAG_* values. Therefore, when converting over to meson
> for app build correctness the "-march=" flag was included in the
> pkg-config cflags output.
> 
> However, since the DPDK-specific CPU flags are now obsolete, and we
> instead check directly for compiler-defined flags, we should no longer
> need to force the exact same architecture match in all cases. To
> faciliate such flexibility, a new pkg_config_machine_args array - which
> defaults to the existing machine_args array has been defined. The
> individual architectures - x86, arm and ppc - can choose if and how to
> override this value themselves.
> 
> For x86, since SSE4.2 is the minimum instruction-set level needed to run
> DPDK, and since some header files assume that minimum level of
> instruction set support, we override the "-march=" value with "-msse4"
> for the pkg-config file. This allows end applications to set their own
> "march" value while still ensuring valid DPDK compilation.
> 
> Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> ---
> 
> CC: Jerin Jacob <jerinj@marvell.com>
> CC: Ruifeng Wang <ruifeng.wang@arm.com>
> CC: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> CC: David Christensen <drc@linux.vnet.ibm.com>
> Feedback requested from ARM and PPC maintainers as to this change
> and what flags, if any, need to be in the .pc file for DPDK on such
> platforms. For example - is setting 'pkg_config_machine_args' to
> 'machine_args' by default necessary behaviour?
> 

Ping for further thoughts or input on this patch.

/Bruce

> ---
>  buildtools/pkg-config/meson.build | 2 +-
>  config/meson.build                | 1 +
>  config/x86/meson.build            | 3 +++
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/buildtools/pkg-config/meson.build b/buildtools/pkg-config/meson.build
> index 5f1930428..8e2e24681 100644
> --- a/buildtools/pkg-config/meson.build
> +++ b/buildtools/pkg-config/meson.build
> @@ -2,7 +2,7 @@
>  # Copyright(c) 2020 Intel Corporation
> 
>  pkg = import('pkgconfig')
> -pkg_extra_cflags = ['-include', 'rte_config.h'] + machine_args
> +pkg_extra_cflags = ['-include', 'rte_config.h'] + pkg_config_machine_args
>  if is_freebsd
>  	pkg_extra_cflags += ['-D__BSD_VISIBLE']
>  endif
> diff --git a/config/meson.build b/config/meson.build
> index 3ddcc3539..d58ff781d 100644
> --- a/config/meson.build
> +++ b/config/meson.build
> @@ -100,6 +100,7 @@ if host_machine.cpu_family().startswith('ppc')
>  else
>  	machine_args += '-march=' + machine
>  endif
> +pkg_config_machine_args = machine_args # may be overriden late per arch
> 
>  toolchain = cc.get_id()
>  dpdk_conf.set_quoted('RTE_TOOLCHAIN', toolchain)
> diff --git a/config/x86/meson.build b/config/x86/meson.build
> index 31bfa63b1..fcd75502e 100644
> --- a/config/x86/meson.build
> +++ b/config/x86/meson.build
> @@ -16,6 +16,9 @@ if cc.get_define('__SSE4_2__', args: machine_args) == ''
>  	machine_args += '-msse4'
>  endif
> 
> +# require SSE4.2 in pkg-config file - don't use -march
> +pkg_config_machine_args = ['-msse4']
> +
>  base_flags = ['SSE', 'SSE2', 'SSE3','SSSE3', 'SSE4_1', 'SSE4_2']
>  foreach f:base_flags
>  	compile_time_cpuflags += ['RTE_CPUFLAG_' + f]
> --
> 2.27.0
>