Re: [dpdk-dev] [PATCH v5 2/2] net/hns3: refactor SVE code compile method

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 5/18/2021 4:48 PM, Honnappa Nagarahalli wrote:
> <snip>
>>
>> On 5/14/2021 10:53 AM, Chengwen Feng wrote:
>>> 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
>>>
>>
>> Hi Chengwen,
>>
>> As far as I understand from above problem statement, you want to produce a
>> binary that can run in two different platforms, one supports only NEON
>> instructions, other supports NEON + SVE.
>>
>> For this driver should be compiled in a way to support min instruction set,
>> which is NEON.
>>
>> There are two build items,
>>
>> 1) hns3_rxtx_vec_sve.c
>> 2) rest of the library
>>
>> There is already runtime checks to select Rx/Tx functions, so it is safe to build
>> (1) as long as compiler supports. If the platform doesn't support SVE, the SVE
>> path won't be selected during runtime.
>>
>> For (2), it should be build to support NEON only, if it is compiled to support
>> SVE, it won't run on the platform that only supports NEON.
>>
>> So, in below, if '__ARM_FEATURE_SVE' is supported, all driver is build with
>> SVE support, won't this cause a problem on the NEON platform?
> The first if statement checks if the user has enabled SVE during compilation which indicates that the user will run the binary on a platform that has SVE (the minimum ISA level supported by this binary), hence it is ok to compile all the code with SVE.
> 

So it is related to the what user provided (I assume as compiler flag), instead
of host HW capability.

> If the user has not enabled SVE during compilation which indicates the user might run the binary on a platform that does not have SVE, the second if statement, checks if the compiler supports SVE. If yes, it will compile the SVE version of the driver as well and the run time checks choose the correct version.
> 

OK, this sounds good, thanks for clarification.

>>
>> What do you think to only keep the else leg of the below check, which is if
>> compiler supports SVE, set '-DCC_SVE_SUPPORT' flag and only build (1) with
>> SVE flag?
>>
>>> 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']
>>>          sources += files('hns3_rxtx_vec_sve.c')
>>> +    elif cc.has_argument('-march=armv8.2-a+sve')
>>> +        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')
>>>      endif
>>>  endif
>>>
>