linux-kernel - Re: [PATCH 4/6] x86/irq: Process nmi sources in NMI handler

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Date: Thu, 30 May 2024 09:10:25 -0700
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: X86 Kernel <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Thomas
 Gleixner <tglx@...utronix.de>, Dave Hansen <dave.hansen@...el.com>, Ingo
 Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 linux-perf-users@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
 Andi Kleen <andi.kleen@...el.com>, Xin Li <xin3.li@...el.com>,
 jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH 4/6] x86/irq: Process nmi sources in NMI handler

Hi Peter,

On Wed, 29 May 2024 13:47:09 -0700, "H. Peter Anvin" <hpa@...or.com> wrote:

> On 5/29/24 13:33, Jacob Pan wrote:
> > +
> > +	/*
> > +	 * Per NMI source specification, there is no guarantee that a
> > valid
> > +	 * NMI vector is always delivered, even when the source
> > specified
> > +	 * one. It is software's responsibility to check all available
> > NMI
> > +	 * sources when bit 0 is set in the NMI source bitmap. i.e. we
> > have
> > +	 * to call every handler as if we have no NMI source.
> > +	 * On the other hand, if we do get non-zero vectors, we know
> > exactly
> > +	 * what the sources are. So we only call the handlers with the
> > bit set.
> > +	 */
> > +	if (source_bitmask & BIT(NMI_SOURCE_VEC_UNKNOWN)) {
> > +		pr_warn_ratelimited("NMI received with unknown
> > source\n");
> > +		return 0;
> > +	}
> > +  
> 
> Note: if bit 0 is set, you can process any other bits first (on the 
> general assumption that if you bother with NMI source then those events 
> are performance sensitive), and you could even exclude them from the 
> poll. This is an optimization, and what you have here is correct from a 
> functional point of view.
> 
Yes, it is a good optimization but also a rare case that bit 0 is set. no?

> > +	source_bitmask = fred_event_data(regs);
> > +	if (!source_bitmask) {
> > +		pr_warn_ratelimited("NMI received without source
> > information!\n");
> > +		return 0;
> > +	}  
> 
> If the event data word is 0, it probably should be treated as a 
> *permanent* failure, as it is a Should Not Happen[TM] situation, and 
> means there is an implementation (or, perhaps more likely, 
> virtualization!) bug, and as such it may not be safe to trust the NMI 
> source information in the future.
> 
Good point, I will add a flag to permanently disable NMI source reporting
for this boot cycle if that happens.

> > +	if (!cpu_feature_enabled(X86_FEATURE_NMI_SOURCE) || type !=
> > NMI_LOCAL)
> > +		return 0;  
> 
> I'm not sure I understand why you are requiring type to be NMI_LOCAL here?
> 
It is just for this current implementation I am not including external
NMIs. AFAIK, there is no users, i.e. no device MSIs delivered as NMI. I saw
effort trying to make HPET NMI watchdog but not materialized.

Thanks,

Jacob