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From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [PATCH] x86/hpet: Read HPET directly if panic in progress
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Date: Wed, 29 May 2024 09:42:43 +0200
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Linus!

On Tue, May 28 2024 at 16:22, Linus Torvalds wrote:
> On Tue, 28 May 2024 at 15:12, Thomas Gleixner <tglx@linutronix.de> wrote:
> I see the smiley, but yeah, I don't think we really care about it.

Indeed. But the same problem exists on other architectures as
well. drivers/clocksource alone has 4 examples aside of i8253

>>   1) Should we provide a panic mode read callback for clocksources which
>>      are affected by this?
>
> The current patch under discussion may be ugly, but looks workable.
> Local ugliness isn't necessarily a show-stopper.
>
> So if the HPET is the *only* case which has this situation, I vote for
> just doing the ugly thing.
>
> Now, if *other* cases exist, and can't be worked around in similar
> ways, then that argues for a more "proper" fix.
>
> And no, I don't think i8253 is a strong enough argument. I don't
> actually believe you can realistically find a machine that doesn't
> have HPET or the TSC and really falls back on the i8253 any more. And
> if you *do* find hw like that, is it SMP-capable? And can you find
> somebody who cares?

Probably not.

>>   2) Is it correct to claim that a MCE which hits user space and ends up in
>>      mce_panic() is still just a regular exception or should we upgrade to
>>      NMI class context when we enter mce_panic() or even go as far to
>>      upgrade to NMI class context for any panic() invocation?
>
> I do think that an NMI in user space should be considered mostly just
> a normal exception. From a kernel perspective, the NMI'ness just
> doesn't matter.

That's correct. I don't want to change that at all especially not for
recoverable MCEs.

> That said, I find your suggestion of making 'panic()' just basically
> act as an NMI context intriguing. And cleaner than the
> atomic_read(&panic_cpu) thing.
>
> Are there any other situations than this odd HPET thing where that
> would change semantics?

I need to go and stare at this some more.

Thanks,

        tglx