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

From: H. Peter Anvin
Date: Mon Oct 22 2012 - 12:03:05 EST

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

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.


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

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