Re: [dpdk-stable] AVX512 bug on SkyLake

[Thomas Monjalon <thomas@monjalon.net>]

Hi,

We need to gather more information about this bug.
More below.

07/11/2018 10:04, Wiles, Keith:
> > On Nov 6, 2018, at 9:30 PM, Yongseok Koh <yskoh@mellanox.com> wrote:
> >> 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.
> 
> Are you not turning off all of the GCC versions for AVX512.
> And you can test for range or greater then GCC version and
> it just seems like we are turning off every gcc version, is that true?

Do we know exactly which GCC versions are affected?

> >> 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.
> 
> Still this is too obscure if nothing else give a link to
> a specific bug not just 97.

The URL is
	https://bugs.dpdk.org/show_bug.cgi?id=97
The bug is also pointing to an email:
	https://mails.dpdk.org/archives/dev/2018-September/111522.html

Summary:
	- CPU: Intel Skylake
	- Linux environment: Ubuntu 18.04
	- Compiler: gcc-7.3 (Ubuntu 7.3.0-16ubuntu3)
	- Scenario: testpmd crashes when it starts forwarding
	- Behaviour: AVX2 version of rte_memcpy() optimized with 512b instructions
	- Fix: disable AVX512 optimization with -mno-avx512f

It seems to have been reproduced only when using mlx5 PMD so far.
Any other experience?