linux-kernel - Re: [RFC] x86/mce: Add workaround for SKX/CLX/CPX spurious machine checks

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Date:   Mon, 7 Feb 2022 21:27:43 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Jue Wang <juew@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [RFC] x86/mce: Add workaround for SKX/CLX/CPX spurious machine
 checks

On Mon, Feb 07, 2022 at 11:24:53AM -0800, Luck, Tony wrote:
> I suggested breaking it out as a helper to make the
> code easier to read.

We have waaay too many small helpers. I guess it is just as readable if
you do in the function:

	bool is_intel_srar = mci_status &
			(MCI_STATUS_VAL|MCI_STATUS_OVER|MCI_STATUS_UC|MCI_STATUS_EN|
			(MCI_STATUS_ADDRV|MCI_STATUS_MISCV|MCI_STATUS_PCC|
			 MCI_STATUS_AR|MCI_STATUS_S)) ==
			(MCI_STATUS_VAL|MCI_STATUS_UC|MCI_STATUS_EN|MCI_STATUS_ADDRV|
			 MCI_STATUS_MISCV|MCI_STATUS_AR|MCI_STATUS_S);

> X86_FEATURE_FSRM is a different (but confusingly simlilar) feature.
> 
> The MSR is per-thread. So the write only disabled the fast string
> operation on this one logical CPU. So the per-cpu srar_dcu_signaled
> variable is just to avoid getting into a loop when this #MC isn't
> because of a REP MOVS peeking at things it shouldn't.

In that case, you can just as well test the MSR bit directly
MSR_IA32_MISC_ENABLE_FAST_STRING_BIT. If it set, you clear it, done.

> Maybe this would be more human friendly?
> 
> 		pr_err("CPU%d: Performance now degraded after applying machine check workaround\n",
> 			smp_processor_id());

Well, is there an erratum you can refer to in it instead?

Explaining the whole deal in a single error message is hard and almost
certainly insufficient.

Also, what's the use of that message issuing once on every CPU? Instead
of being a _once() message?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette