linux-kernel - Re: [patch 06/11] x86: nmi_32/64.c - use apic_write_around instead of apic

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Date:	Wed, 28 May 2008 22:32:12 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	hpa@...or.com, tglx@...utronix.de, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 06/11] x86: nmi_32/64.c - use apic_write_around instead
	of apic_write

[Maciej W. Rozycki - Wed, May 28, 2008 at 06:47:56PM +0100]
| On Wed, 28 May 2008, Cyrill Gorcunov wrote:
| 
| > Thanks a lot, Maciej!!! Could you please explain me how did you find
| > that? 'cause reporter said that with nmi_watchdog=2 it works and with
| > nmi_watchdog=1 it stalls? Maybe I should better make this function
| > the same as 64bit version has? I.e. set nmi_watchdog = NMI_NONE by default?
| 
|  Well, nmi_watchdog=1 is the I/O APIC watchdog and if no watchdog has been
| specified at the command line, the piece of code you have moved selects
| between the local and the I/O APIC watchdog based on availability of the
| former.  So in this case the local watchdog must have been unavailable as
| it works if requested explicitly.
| 
|  No piece of code in nmi_watchdog_default() touches peripheral hardware
| and native_smp_prepare_cpus() is called early enough the system is still
| running UP and no APIC setup has happened yet, so any interference with
| running hardware can be excluded.
| 
|  Random lock-ups are a typical symptom of the NMI watchdog interfering
| with SMM firmware -- of course in the context of the watchdog being
| suspected in the first place -- there may be plenty of other reasons of
| random lock-ups.  Obviously this is the SMM firmware asking for trouble
| explicitly, because NMIs are disabled by the processor upon entering the
| SMM and it is the SMI handler that unmasks the NMI explicitly (with an
| IRET, which shouldn't be used in the SMM mode at all) -- otherwise it
| wouldn't even notice the watchdog running, but there you go.
| 
|  As a rule of thumb any piece of firmware that has a possibility to run
| from an OS context should not use interrupts of any kind, because it is
| quite likely it cannot handle them in the way the OS expects them to be
| handled.  It is as simple as that, but perhaps too simple for some to
| comprehend. :(
| 
|   Maciej
| 

oh... :( i think i just restore old behaviour for now. Thanks for explanation!

		- Cyrill -
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