Re: [dpdk-dev] [PATCH 2/2] net/hns3: refactor SVE code compile method

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> >
> > Currently, the SVE code is compiled only when -march supports SVE
> > (e.g. '- march=armv8.2a+sve'), there maybe some problem[1] with this
> approach.
> >
> > The solution:
> > a. If the minimum instruction set support SVE then compiles it.
> > b. Else if the compiler support SVE then compiles it.
> > c. Otherwise don't compile it.
> >
> > [1] https://mails.dpdk.org/archives/dev/2021-April/208189.html
> >
> > Fixes: 8c25b02b082a ("net/hns3: fix enabling SVE Rx/Tx")
> > Fixes: 952ebacce4f2 ("net/hns3: support SVE Rx")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
> > ---
> >  drivers/net/hns3/hns3_rxtx.c |  2 +-
> >  drivers/net/hns3/meson.build | 13 +++++++++++++
> >  2 files changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/hns3/hns3_rxtx.c
> > b/drivers/net/hns3/hns3_rxtx.c index
> > 1d7a769..4ef20c6 100644
> > --- a/drivers/net/hns3/hns3_rxtx.c
> > +++ b/drivers/net/hns3/hns3_rxtx.c
> > @@ -2808,7 +2808,7 @@ hns3_get_default_vec_support(void)
> >  static bool
> >  hns3_get_sve_support(void)
> >  {
> > -#if defined(RTE_ARCH_ARM64) && defined(__ARM_FEATURE_SVE)
> > +#if defined(CC_SVE_SUPPORT)
> >  	if (rte_vect_get_max_simd_bitwidth() < RTE_VECT_SIMD_256)
> >  		return false;
> >  	if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_SVE))
> > diff --git a/drivers/net/hns3/meson.build
> > b/drivers/net/hns3/meson.build index
> > 53c7df7..8563d70 100644
> > --- a/drivers/net/hns3/meson.build
> > +++ b/drivers/net/hns3/meson.build
> > @@ -35,7 +35,20 @@ deps += ['hash']
> >
> >  if arch_subdir == 'arm' and dpdk_conf.get('RTE_ARCH_64')
> >      sources += files('hns3_rxtx_vec.c')
> > +
> > +    # compile SVE when:
> > +    # a. support SVE in minimum instruction set baseline
> > +    # b. it's not minimum instruction set, but compiler support
> >      if cc.get_define('__ARM_FEATURE_SVE', args: machine_args) != ''
> > +        cflags += ['-DCC_SVE_SUPPORT']
> Why is the CC_SVE_SUPPORT flag needed? The compiler has
> __ARM_FEATURE_SVE flag already which gets defined when '+sve" is added to
> '-march'.
> 
> >          sources += files('hns3_rxtx_vec_sve.c')
> > +    elif cc.has_argument('-march=armv8.2-a+sve')
> I think this check and the above check do the same thing. i.e. both of them
> check if +sve flag is passed to the compiler.
> 
> > +        cflags += ['-DCC_SVE_SUPPORT']
> > +        hns3_sve_lib = static_library('hns3_sve_lib',
> > +                        'hns3_rxtx_vec_sve.c',
> > +                        dependencies: [static_rte_ethdev],
> > +                        include_directories: includes,
> > +                        c_args: [cflags, '-march=armv8.2-a+sve'])
> > +        objs += hns3_sve_lib.extract_objects('hns3_rxtx_vec_sve.c')
I do not understand the need of this block of code, appreciate if you could explain why this is required.

> >      endif
> >  endif
> > --
> > 2.8.1