linux-kernel - Re: [PATCH v6 4/9] x86/nmi: Assign and register NMI-source vectors

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Message-ID: <e978e1fb-d88e-4789-bd33-367281dfa0ad@intel.com>
Date: Tue, 3 Jun 2025 10:23:42 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Cc: Xin Li <xin@...or.com>, "H . Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Kan Liang <kan.liang@...ux.intel.com>, Tony Luck <tony.luck@...el.com>,
 Zhang Rui <rui.zhang@...el.com>, Steven Rostedt <rostedt@...dmis.org>,
 Andrew Cooper <andrew.cooper3@...rix.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Jacob Pan <jacob.pan@...ux.microsoft.com>, Andi Kleen <ak@...ux.intel.com>,
 Kai Huang <kai.huang@...el.com>, Sandipan Das <sandipan.das@....com>,
 linux-perf-users@...r.kernel.org, linux-edac@...r.kernel.org,
 kvm@...r.kernel.org, linux-pm@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v6 4/9] x86/nmi: Assign and register NMI-source vectors

On 5/13/25 13:37, Sohil Mehta wrote:
...
> + * Vector 2 is reserved for external NMIs related to the Local APIC -
> + * LINT1. Some third-party chipsets may send NMI messages with a
> + * hardcoded vector of 2, which would result in bit 2 being set in the
> + * NMI-source bitmap.

This doesn't actually say what problem this causes. Is this better?

	Third-party chipsets send NMI messages with a fixed vector of 2.
	Using vector 2 for some other purpose would cause confusion
	between those Local APIC messages and the other purpose. Avoid
	using it.

> + * The vectors are in no particular priority order. Add new vector
> + * assignments sequentially in the list below.
> + */
> +#define NMIS_VECTOR_NONE	0	/* Reserved - Set for all unidentified sources */
> +#define NMIS_VECTOR_TEST	1	/* NMI selftest */
> +#define NMIS_VECTOR_EXTERNAL	2	/* Reserved - Match External NMI vector 2 */
> +#define NMIS_VECTOR_SMP_STOP	3	/* Panic stop CPU */
> +#define NMIS_VECTOR_BT		4	/* CPU backtrace */
> +#define NMIS_VECTOR_KGDB	5	/* Kernel debugger */
> +#define NMIS_VECTOR_MCE		6	/* MCE injection */
> +#define NMIS_VECTOR_PMI		7	/* PerfMon counters */
> +
> +#define NMIS_VECTORS_MAX	16	/* Maximum number of NMI-source vectors */

Would an enum fit here?

You could also add a:

	NMIS_VECTOR_COUNT

as the last entry and then just:

	BUILD_BUG_ON(NMIS_VECTOR_COUNT >= 16);

somewhere.

I guess it's a little annoying that you need NMIS_VECTOR_EXTERNAL to
have a fixed value of 2, but I do like way the enum makes the type explicit.

>  static int __init register_nmi_cpu_backtrace_handler(void)
>  {
> -	register_nmi_handler(NMI_LOCAL, nmi_cpu_backtrace_handler, 0, "arch_bt", 0);
> +	register_nmi_handler(NMI_LOCAL, nmi_cpu_backtrace_handler, 0, "arch_bt", NMIS_VECTOR_BT);
>  	return 0;
>  }

... Oh you replaced _most_ of the random 0's in this patch. That helps
for sure.