linux-kernel - Re: [PATCH 1/2] boot: ignore early NMIs

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Date:	Tue, 13 Mar 2012 09:33:50 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-tip-commits@...r.kernel.org, torvalds@...ux-foundation.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com, tglx@...utronix.de, mingo@...e.hu,
	Yinghai Lu <yinghai@...nel.org>, akpm@...ux-foundation.org,
	vgoyal@...hat.com
Subject: Re: [PATCH 1/2] boot: ignore early NMIs

On Tue, Mar 13, 2012 at 11:11:49AM +0900, Fernando Luis Vázquez Cao wrote:
> On 03/13/2012 05:16 AM, H. Peter Anvin wrote:
> >On 03/12/2012 01:04 PM, H. Peter Anvin wrote:
> >>On 03/12/2012 01:01 PM, Eric W. Biederman wrote:
> >>>The basic problem is which source do we block this at?  How many
> >>>sources are their?  And architecturally last I looked x86 no longer
> >>>has a NMI disable EFI and similar systems want to get away without
> >>>a CMOS legacy clock because designers so often get them wrong.
> >>>
> >>On all processors which have an LAPIC you can block all NMI sources at
> >>the LAPIC.  I think it's safe to assume that if you don't have an LAPIC
> >>-- an ancient system by now -- you have port 70h.
> >>
> >One thing: *disabling* the LAPIC will allow external NMIs coming in on
> >LINT1 through, since the LAPIC in the disabled state tries to mimic the
> >no-LAPIC configuration.  So I don't think you want to disable LAPIC as
> >much as disable the interrupt vectors within.
> 
> Does this sound like a plan to get the ball rolling?:
> 
> 1.- Merge Don's patch to disable the LAPIC in kdump reboot path (this
>   fixes a real issue seen in the field, is a net win and certainly not a
>   regression - indeed it makes the code simpler because the I/O
>   APICs are left untouched).

I think you mean my patch to stop disabling the I/O APIC.  That patch
hasn't seen any new issues.  It was the piece that stopped disabling the
LAPIC that opened the doors for NMIs to fault the system.

> 
> 2.- Merge my patch set to ignore early NMIs (this brings the behavior
>   of the boot code in line with what we do in the rest of the kernel
>   a we can avoid situations were a spurious NMI causes the kernel
>   to halt). The early NMI handler is temporary and the final NMI
>   handler installed shortly afterwards will take care of subsequent
>   NMIs.
> 
> 3.- Make sure that spurious NMIs (i.e. NMIs that for whatever reason
>   could not be stopped at the source) received during the reboot
>   path to the kdump kernel do not cause a triple fault or a system
>   lockup. This is under testing.

This will require changes in kexec-tools as the purgatory code zaps the
GDT I believe.  This is going to make a 'complete solution' dependent on
a version of kexec-tools.  Not sure what we want to do there.

> 
> 4.- Identify all the NMI sources and keep them from reaching the CPU
>   when it can be done in a race-free way.

Cheers,
Don
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