linux-kernel - Re: [PATCH 9/11] Panic delay fix

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Date:	Tue, 06 Feb 2007 13:59:09 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Andi Kleen <ak@....de>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH 9/11] Panic delay fix

Andi Kleen wrote:
> On Mon, Feb 05, 2007 at 07:53:30PM -0800, Zachary Amsden wrote:
>   
>> Failure to use real-time delay here causes the keyboard to become demonically
>> possessed in the event of a kernel crash, with wildly blinking lights and
>> unpredictable behavior.  This has resulted in several injuries.
>>     
>
> There must be a reason why it wasn't default before. Has this
> reason changed?
>   

This only matters under paravirt; non-paravirt kernels and kernels 
running on native hardware will always behave properly.

But paravirtualized kernels with fake devices have no need to udelay to 
accommodate slow hardware - the hardware is just virtual.  The 
USE_REAL_TIME_DELAY define allows udelay to be specifically reverted 
back to being a real delay.  There are only a couple cases where it 
matters - one is booting APs on SMP systems (there is a real delay 
before they come up), and one is any hardware that drives world 
interacting devices - such as keyboard LEDs in a panic loop.

Zach
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