Re: [dpdk-dev] [EXT] Re: [PATCH] config/arm: add ability to express arch extensions

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

>05/05/2021 14:14, pbhagavatula@marvell.com:
>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>
>> The ARM architecture allows SoCs to have extensions in addition
>> to base profiles such as Large System Extension (LSE), CRC etc.
>>
>> Add ability to declare SoC specific extensions.
>>
>> Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
>
>I don't know what it is fixing. It is an optimization?
>Is it candidate for next release?

Its more of an enhancement, the default n2 march flag doesn't describe 
anything about the extensions that a specific SoC has. 
In case of OCTEON 10 SoC it has support for LSE and CRC in additional to 
the basic n2 profile. 
This patch allows SoC to add extensions to the -march flag.
Default n2 march :    -march=armv8.5-a+crypto+sve2
OCTEON 10 march :  -march=armv8.5-a+crypto+sve2+lse+crc

Including extensions in march helps compiler generate better code.
Example adding '+lse' tells the compiler to inline C11 atomics rather than
having a run time jump

https://gcc.godbolt.org/z/8hPv87dbr

I don't think this is limited to OCTEON 10 other SoC should also add their
extensions to march.

>
>> ---
>>
>>  More details about ARM extensions
>>  https://urldefense.proofpoint.com/v2/url?u=https-
>3A__developer.arm.com_documentation_102378_0200&d=DwICAg&c
>=nKjWec2b6R0mOyPaz7xtfQ&r=1cjuAHrGh745jHNmj2fD85sUMIJ2IPIDs
>IJzo6FN6Z0&m=yCt4te5_7VJvaJT5OXrOpFXH3UBOUqRDlHZuSVEUZHM
>&s=DquFBMwxMpjxWcM2Qw1c2t_ak-j6DN4rz8ce_I43RJk&e=
>>
>>  config/arm/meson.build | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/config/arm/meson.build b/config/arm/meson.build
>> index 22cd81319..8aa961e5b 100644
>> --- a/config/arm/meson.build
>> +++ b/config/arm/meson.build
>> @@ -230,6 +230,7 @@ soc_cn10k = {
>>          ['RTE_MAX_LCORE', 24],
>>          ['RTE_MAX_NUMA_NODES', 1]
>>      ],
>> +    'extensions' : ['lse', 'crc'],
>>      'part_number': '0xd49',
>>      'numa': false
>>  }
>> @@ -387,6 +388,7 @@ else
>>      endif
>>
>>      soc_flags = []
>> +    soc_extensions = []
>>      if soc_config.has_key('not_supported')
>>          error('SoC @0@ not supported.'.format(soc))
>>      elif soc_config != {}
>> @@ -394,6 +396,7 @@ else
>>          implementer_config = implementers[implementer_id]
>>          part_number = soc_config['part_number']
>>          soc_flags = soc_config.get('flags', [])
>> +        soc_extensions = soc_config.get('extensions', [])
>>          if not soc_config.get('numa', true)
>>              has_libnuma = 0
>>          endif
>> @@ -431,6 +434,11 @@ else
>>      # apply supported machine args
>>      machine_args = [] # Clear previous machine args
>>      foreach flag: part_number_config['machine_args']
>> +        if flag.startswith('-march') and soc_extensions.length() != 0
>
>Why condition on start with -march?

There are some implementer flags which don't have -march to add extensions to.

>
>> +            foreach ex: soc_extensions
>> +                flag += '+' + ex
>> +            endforeach
>> +        endif
>
>