linux-kernel - Re: Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.

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Date:	Wed, 09 Jan 2008 13:53:34 -0800
From:	Zachary Amsden <zach@...are.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"David P. Reed" <dpreed@...d.com>, Avi Kivity <avi@...amnet.com>,
	Christer Weinigel <christer@...nigel.se>,
	Ondrej Zary <linux@...nbow-software.org>,
	Rene Herman <rene.herman@...access.nl>,
	Bodo Eggert <7eggert@....de>, Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Paul Rolland <rol@...917.net>, Pavel Machek <pavel@....cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	rol <rol@...be.net>
Subject: Re: Re: [PATCH] x86: provide a DMI based port 0x80
	I/O delay override.

On Tue, 2008-01-08 at 21:19 -0800, H. Peter Anvin wrote:
> Zachary Amsden wrote:
> > 
> > BTW, it isn't ever safe to pass port 0x80 through to hardware from a
> > virtual machine; some OSes use port 0x80 as a hardware available scratch
> > register (I believe Darwin/x86 did/does this during boot).
> 
> That's funny, because there is definitely no guarantee that you get back 
> what you read (well, perhaps there is on Apple.)

According to Phoenix Technologies book "System BIOS for IBM PCs,
Compatibles and EISA Computers, 2nd Edition", the I/O port list gives

port 0080h   R/W  Extra page register (temporary storage)

Despite looking, I've never seen it documented anywhere else, but I
believe it works on just about every PC platform.  Except, apparently,
my laptop.

Zach

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