linux-kernel - Re: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP

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Date:	Mon, 22 Oct 2012 09:02:06 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
CC:	fenghua.yu@...el.com, linux-kernel@...r.kernel.org,
	kexec@...ts.infradead.org, x86@...nel.org, mingo@...e.hu,
	tglx@...utronix.de, len.brown@...el.com, vgoyal@...hat.com,
	ebiederm@...ssion.com, grant.likely@...retlab.ca,
	rob.herring@...xeda.com
Subject: Re: [PATCH v1 0/2] x86, apic: Disable BSP if boot cpu is AP

On 10/15/2012 11:38 PM, HATAYAMA Daisuke wrote:
>
> Thanks for pointing out this. And I've recalled my investigation in
> the past now. So I want to stop retrying your patch v9 now. This NMI
> method is definitely not applicable to 2nd kernel without any change.
>
> Your NMI method assumes BSP thread is halting in play dead loop. But
> on the 2nd kernel, BSP is halting in the 1st kernel (or possibly in a
> fatail system error). Even if throwing NMI to BSP, it goes back to the
> 1st kernel soon again. I at least confirmed NMI handler is executed in
> this case.
>
> Also, throwing NMI changes stack in the 1st kernel, which is
> unpermissible from kdump's perspective. But x86_64 uses Interrupt
> Stack Table (IST), in which stack switching is not performed. So 2nd
> kernel's stack is used at least on x86_64.
>
> To sum up, to apply NMI method in the 2nd kernel, I think it necessary
> to modify contexts pushed on the stack so execution goes to the 2nd
> kernel's start_secondary() while initializing its state
> appropreately.
>
> Also I think it necessary to discuss whether this NMI method is
> reliable enough for kdump use.
>

I think it's pretty clear it is *not*.  NMI or monitor would either have 
to rely on context set up by the first kernel, which simply isn't safe. 
  Out of those two options, a monitor would actually be safer, since it 
can be self-contained in a completely different way.

However, it seems that running on N-1 CPUs in kdump is perfectly acceptable.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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