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Date:   Wed, 17 Mar 2021 11:13:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Kim Phillips <kim.phillips@amd.com>, Jiri Olsa <jolsa@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Tom Lendacky <thomas.lendacky@amd.com>, x86@kernel.org,
        lkml <linux-kernel@vger.kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Stanislav Kozina <skozina@redhat.com>,
        Michael Petlan <mpetlan@redhat.com>,
        Pierre Amadio <pamadio@redhat.com>, onatalen@redhat.com,
        darcari@redhat.com, "Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: unknown NMI on AMD Rome
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On Wed, Mar 17, 2021 at 09:48:29AM +0100, Ingo Molnar wrote:
> > https://developer.amd.com/wp-content/resources/56323-PUB_0.78.pdf
> 
> So:
> 
> 
>   1215 IBS (Instruction Based Sampling) Counter Valid Value
>   May be Incorrect After Exit From Core C6 (CC6) State
> 
>   Description
> 
>   If a core's IBS feature is enabled and configured to generate an interrupt, including NMI (Non-Maskable
>   Interrupt), and the IBS counter overflows during the entry into the Core C6 (CC6) state, the interrupt may be
>   issued, but an invalid value of the valid bit may be restored when the core exits CC6.
>   Potential Effect on System
> 
>   The operating system may receive interrupts due to an IBS counter event, including NMI, and not observe an
>   valid IBS register. Console messages indicating "NMI received for unknown reason" have been observed on
>   Linux systems.
> 
>   Suggested Workaround: None
>   Fix Planned: No fix planned

Should be simple enough to disable CC6 while IBS is in use. Kim, can you
please make that happen?