linux-kernel - Re: [PATCH], issue EOI to APIC prior to calling crash_kexec in die

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Date:	Thu, 7 Feb 2008 01:39:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Vivek Goyal <vgoyal@...hat.com>,
	Neil Horman <nhorman@...driver.com>, tglx@...utronix.de,
	mingo@...hat.com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH], issue EOI to APIC prior to calling crash_kexec in
	die_nmi path


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Looking at the patch the local_irq_enable() is totally bogus.  As soon 
> was we hit machine_crash_shutdown the first thing we do is disable 
> irqs.

yeah.

> I'm wondering if someone was using the switch cpus on crash patch that 
> was floating around.  That would require the ipis to work.
> 
> I don't know if nmi_exit makes sense.  There are enough layers of 
> abstraction in that piece of code I can't quickly spot the part that 
> is banging the hardware.
> 
> The location of nmi_exit in the patch is clearly wrong.  crash_kexec 
> is a noop if we don't have a crash kernel loaded (and if we are not 
> the first cpu into it), so if we don't execute the crash code 
> something weird may happen.  Further the code is just more 
> maintainable if that kind of code lives in machine_crash_shutdown.

nmi_exit() has no hw effects - it's just our own bookeeping.

the hw knows that we finished the NMI when we do an iret. Perhaps that's 
the bug or side-effect that made the difference: via enabling irqs we 
get an irq entry, and that does an iret and clears the NMI nested state 
- allowing the kexec context to proceed? I suspect kexec() will do an 
iret eventually (at minimum in the booted up kernel's context) - all 
NMIs are blocked up to that point and maybe the APIC doesnt really like 
being frobbed in that state? In any case, the local_irq_enable() is just 
wrong - it's the worst thing a crashing kernel can do. Perhaps doing an 
intentional iret with a prepared stack-let that just restores to 
still-irqs-off state and jumps to the next instruction could 'exit' the 
NMI context without really having to exit it in the kernel code flow?

	Ingo
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