linux-kernel - Re: [PATCH v5 5/9] x86/nmi: Add support to handle NMIs with source information

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Message-ID: <20250508204906.GJ4439@noisy.programming.kicks-ass.net>
Date: Thu, 8 May 2025 22:49:06 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Xin Li <xin@...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>,
	Sean Christopherson <seanjc@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Tony Luck <tony.luck@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Zhang Rui <rui.zhang@...el.com>, Lukasz Luba <lukasz.luba@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Brian Gerst <brgerst@...il.com>,
	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>,
	Nikolay Borisov <nik.borisov@...e.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 v5 5/9] x86/nmi: Add support to handle NMIs with source
 information

On Thu, May 08, 2025 at 01:23:04PM -0700, H. Peter Anvin wrote:
> On May 8, 2025 5:15:44 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:

> >> Looks good, except when fred_event_data() returns 0. I don't expect it
> >> to happen in practice. But, maybe with new hardware and eventually
> >> different hypervisors being involved, it is a possibility.
> >> 
> >> We can either call it a bug that an NMI happened without source
> >> information. Or be extra nice and do this:
> >> 
> >> if (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE) && type == NMI_LOCAL) {
> >> 	source = fred_event_data(regs);
> >> 	if (!source || (source & BIT(0)))
> >> 		source = ~0UL;
> >> }
> >
> >Perhaps also WARN about the !source case?
> 
> A 0 should be interpreted such that NMI source is not available, e.g.
> due to a broken hypervisor or similar.

I'm reading that as an agreement for WARN-ing on 0. We should definitely
WARN on broken hypervisors and similar.