linux-kernel - Re: reason for delay in arch/x86/kernel/traps.c::io_check

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Message-ID: <20090310153242.GA23463@elte.hu>
Date:	Tue, 10 Mar 2009 16:32:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Chris Friesen <cfriesen@...tel.com>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: reason for delay in arch/x86/kernel/traps.c::io_check_error()?


* Chris Friesen <cfriesen@...tel.com> wrote:

> Hi all,
>
> I was just wondering about the basis for the delay in 
> io_check_error().  The ICH7 manual doesn't have any mention of 
> a delay being required here--is it necessary for other 
> hardware, something not mentioned in the manual, or just an 
> accident?

That code has seriously bitrotten along the years. All those 
port 61H accesses:

arch/x86/kernel/traps.c:                reason = get_nmi_reason();
arch/x86/kernel/traps.c:        outb(reason, 0x61);
arch/x86/kernel/traps.c:        outb(reason, 0x61);
arch/x86/kernel/traps.c:        outb(reason, 0x61);

... are often wrong on modern chipsets - including the logic in 
io_check_error(). But we dont really have lowlevel chipset 
drivers on this level in Linux, so there's nothing suitable to 
replace it with and it never got fixed.

Can you see this trigger on a box perhaps? Or are you worried 
about the potential unbound execution time of this function 
which can be up to 2 seconds in NMI context?

	Ingo
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