linux-kernel - Re: [PATCH 1/2] VMware detection support for x86 and x86-64

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Date:	Wed, 24 Sep 2008 22:30:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	akataria@...are.com
CC:	Alok kataria <alokkataria1@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Yan Li <elliot.li.tech@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"joerg.roedel@....com" <joerg.roedel@....com>,
	"rjmaomao@...il.com" <rjmaomao@...il.com>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Hecht <dhecht@...are.com>, Zach Amsden <zach@...are.com>
Subject: Re: [PATCH 1/2] VMware detection support for x86 and x86-64

Alok Kataria wrote:
> 
> Hmm...what can a IN on an unknown port cause on native hardware, if a
> port is not being used it would return 0xFFFFFFFF in eax, and if you
> have a real device there (a sane one), what can IN result in apart from
> reading some IO register/counter value in eax ?
> If there is anything apart from the above 2 outcomes, please let me know
> exactly what you mean.
> 

First, you are assuming all devices are "sane".  This is obviously wrong 
-- you're poking in hyperspace, and you don't know if you're going to 
hit someone's ancient controller card that perhaps drives a medical 
accelerator for all you know.

Second, you are assuming that devices you call "sane" don't have I/O 
ports with read side effects.  Many, if not most, devices have some I/O 
ports with read side effects, especially read-clear semantics and/or 
queue drain operations.

Third, in the real world hardware is buggy.  Not just a little, but 
severely so.  Accessing a part of a device which is uninitialized, 
powered down or plain broken can wedge the device or the whole system.

In short, poking at I/O ports which you don't know what they are at best 
takes us bad to the bad old days of ISA probing (without the protection 
of customary address assignments); I think it has to be an absolutely 
last resort and would be reflective of utterly incompetent design.  It 
is significantly *worse* than stealing random opcodes, Virtual PC-style, 
and that is also unacceptable.

	-hpa
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