Re: [dpdk-dev] [PATCH v2] build: disable compiler AVX512F support

[Yongseok Koh <yskoh@mellanox.com>]

> On Nov 5, 2018, at 6:06 AM, Wiles, Keith <keith.wiles@intel.com> wrote:
> 
> 
> 
>> On Nov 2, 2018, at 9:04 PM, Yongseok Koh <yskoh@mellanox.com> wrote:
>> 
>> This is a workaround to prevent a crash, which might be caused by
>> optimization of newer gcc (7.3.0) on Intel Skylake.
> 
> Should the code below not also test for the gcc version and the Sky Lake processor, maybe I am wrong but it seems it is turning AVX512 for all GCC builds

I didn't want to check gcc version as 7.3.0 is very new. Only gcc 8 is newly up since then (gcc 8.2).
Also, I wasn't able to test every gcc versions and I wanted to be a bit conservative for this crash.
Performance drop (if any) by disabling a new (experimental) feature would be less risky than unaccountable crash.
And, it does disable the feature only if CONFIG_RTE_ENABLE_AVX512=n. Please refer to v3.

> 
> Also bug 97 seems a bit obscure reference, maybe you know the bug report, but more details would be good?

I sent out the report to dev list two month ago. And I created the Bug 97 in order to reference it in the commit message.
I didn't want to repeat same message here and there, but it would've been better to have some sort of summary of the Bug, although v3 has a few more words. However, v3 has been merged.

Thanks,
Yongseok

>> 
>> Bugzilla ID: 97
>> 
>> Cc: stable@dpdk.org
>> 
>> Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
>> ---
>> 
>> v2:
>> * disable the flag only in case of gcc
>> 
>> config/x86/meson.build | 5 +++++
>> mk/rte.cpuflags.mk     | 7 +++++++
>> 2 files changed, 12 insertions(+)
>> 
>> diff --git a/config/x86/meson.build b/config/x86/meson.build
>> index 33efb5e547..8ddca0ea9f 100644
>> --- a/config/x86/meson.build
>> +++ b/config/x86/meson.build
>> @@ -47,6 +47,11 @@ endif
>> if cc.get_define('__AVX512F__', args: march_opt) != ''
>> 	dpdk_conf.set('RTE_MACHINE_CPUFLAG_AVX512F', 1)
>> 	compile_time_cpuflags += ['RTE_CPUFLAG_AVX512F']
>> +else
>> +# disable AVX512F support of gcc as a workaround for Bug 97
>> +	if cc.get_id() == 'gcc' and cc.has_argument('-mavx512f')
>> +		machine_args += '-mno-avx512f'
>> +	endif
>> endif
>> 
>> dpdk_conf.set('RTE_CACHE_LINE_SIZE', 64)
>> diff --git a/mk/rte.cpuflags.mk b/mk/rte.cpuflags.mk
>> index 43ed84155b..a8c26fb011 100644
>> --- a/mk/rte.cpuflags.mk
>> +++ b/mk/rte.cpuflags.mk
>> @@ -68,6 +68,13 @@ endif
>> ifneq ($(filter $(AUTO_CPUFLAGS),__AVX512F__),)
>> ifeq ($(CONFIG_RTE_ENABLE_AVX512),y)
>> CPUFLAGS += AVX512F
>> +else
>> +# disable AVX512F support of gcc as a workaround for Bug 97
>> +ifeq ($(CONFIG_RTE_TOOLCHAIN_GCC),y)
>> +	ifeq ($(shell $(CC) --target-help | grep -q mavx512f && echo 1), 1)
>> +		MACHINE_CFLAGS += -mno-avx512f
>> +	endif
>> +endif
>> endif
>> endif
>> 
>> -- 
>> 2.11.0
>> 
> 
> Regards,
> Keith