linux-kernel - Re: [linus:master] [x86/cpu] f388f60ca9: BUG:soft_lockup-CPU##stuck

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Message-Id: <963683ea-eaba-4430-bd8a-1b11affe104a@app.fastmail.com>
Date: Sun, 27 Apr 2025 14:07:09 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "kernel test robot" <oliver.sang@...el.com>
Cc: oe-lkp@...ts.linux.dev, "kernel test robot" <lkp@...el.com>,
 linux-kernel@...r.kernel.org, "Ingo Molnar" <mingo@...nel.org>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>,
 "John Stultz" <jstultz@...gle.com>, "Thomas Gleixner" <tglx@...utronix.de>,
 "Stephen Boyd" <sboyd@...nel.org>, "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [linus:master] [x86/cpu]  f388f60ca9:
 BUG:soft_lockup-CPU##stuck_for#s![swapper:#]

On Sun, Apr 27, 2025, at 07:48, Oliver Sang wrote:
> On Thu, Apr 24, 2025 at 09:59:38AM +0200, Arnd Bergmann wrote:
>> 
>> Thanks for confirming. So a 486-targeted kernel still passes
>> your tests on modern hardware if we force TSC and CX8 to
>> be enabled, but the boot fails if the options are turned
>> off in Kconfig (though available in emulated hardware).
>> 
>> To be completely sure, you could re-run the same test with
>> just one of these enabled, but I'm rather sure that the TSC
>> is the root cause.
>
> just FYI. we rerun the tests. if only enable X86_TSC, the config diff is

...
> the various issues still exists:

>
> if only enable X86_CMPXCHG64:
...
> various issues gone:

Interesting, so cx8 was indeed a problem. I would still assume
that TSC caused the boot panic I cited, but it looks like CX8
caused all the other symptoms.

At the minimum this strengthens the case for the consensus of
dropping support for all pre-586 cores.

Thanks a lot for confirming! 

      Arnd