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

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Message-ID: <20090412181239.GE6942@lenovo>
Date:	Sun, 12 Apr 2009 22:12:39 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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

[Ingo Molnar - Sun, Apr 12, 2009 at 08:02:39PM +0200]
| 
| * 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
|

Ugh... I expected something like "No, look at that code! You've
missed this snippet!". Which would make me happy :-)

I'll take a look.

        Cyrill
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