linux-kernel - Re: [rfc 0/4] x86 - apic dummy ops with cleanups

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Message-ID: <20090412180239.GA3489@elte.hu>
Date:	Sun, 12 Apr 2009 20:02:39 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	hpa@...or.com, tglx@...utronix.de, xemul@...nvz.org,
	yhlu.kernel@...il.com, linux-kernel@...r.kernel.org,
	macro@...ux-mips.org, andi@...stfloor.org
Subject: Re: [rfc 0/4] x86 - apic dummy ops with cleanups


* Cyrill Gorcunov <gorcunov@...il.com> wrote:

> Thanks, Ingo!
> 
> Actually I found one a bit suspicious thing (not sure if it was 
> initially designed to behave like that).
> 
> Suppose we have SMP compiled kernel and even supported on HW level 
> but then we disable apic thru kernel option. And then when system 
> startup it should be working fine since we go to uniprocessor 
> mode. Then the machine going for reboot and we have:
> 
> Starting from arch/x86/kernel/reboot.c
> 
> native_machine_shutdown
> 	smp_send_stop
> 		smp_ops.smp_send_stop
> 			native_smp_send_stop
> 				disable_local_APIC
> 					apic_write(APIC_SPIV, value);
> 
> Since we have reserved APIC memory mapped area (in _anycase_) it 
> doesn't cause any problems but was it initially supposed?

i dont think that was supposed so. If the APIC wasnt even enabled 
then we shouldnt try to disable it. Note that there are a few 
unusual paths to APIC enablement: for example by twiddling the 
nmi_watchdog knob in /proc/sys - or in theory through a CPU hotplug 
event.

	Ingo
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