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

[fengchengwen <fengchengwen@huawei.com>]

On 2021/5/13 7:21, Honnappa Nagarahalli wrote:
> <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'.
>>

The CC_SVE_SUPPORT is used to implement the hns3_get_sve_support API (below),
this API located in another file which is hns3_rxtx.c, and this file was
compiled with default machine_args.

	static bool
	hns3_get_sve_support(void)
	{
	#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))
			return true;
	#endif
		return false;
	}

If the machine_args doesn't support SVE but compiler support, we will compile
SVE code too, so in this case we need define CC_SVE_SUPPORT, so that in runtime
we could try judge whether support SVE. In this case, __ARM_FEATURE_SVE was not
defined because it is in hns3_rxtx.c which use default machine_args.

If the machine_args supports SVE we sure compile SVE code, in this case, to
maintain consistency, we also define this macro.

>>>          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.
>>

Yes it is.

>>> +        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.
> 

This is typical usage for compiling some soure file with custom flags (here is '-march=armv8.2-a+sve')
Please ref [1] with keyword 'extract_objects'

[1] https://mesonbuild.com/Reference-manual.html

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