Messages in this thread | | | Date | Wed, 24 Sep 2008 22:30:54 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 1/2] VMware detection support for x86 and x86-64 |
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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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