[dpdk-dev] [EXT] Re: [PATCH] config/arm: add ability to express arch extensions
Honnappa Nagarahalli
Honnappa.Nagarahalli at arm.com
Mon May 10 19:20:54 CEST 2021
<snip>
>
> >05/05/2021 14:14, pbhagavatula at marvell.com:
> >> From: Pavan Nikhilesh <pbhagavatula at 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 at 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
Can you compile the code with armv8.5-a instead of armv8-a? I see that LSE instructions are used.
https://gcc.godbolt.org/z/18MYhY7xv
>
> 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
> >
> >
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