Re: Suggestions for an improvement to the man page "iopl - change I/O privilege level".

[TO += Ingo and tlgx]

Hi Daniël, Thomas, Ingo,

quoted

Date: 2026-08-11 17:19:31+0000
From: Daniël Hörchner <redacted>

quoted

Hello,


I have some feedback and suggestions to improve the man page of
"iopl - change I/O privilege level".
 
The purpose of iopl() is not clearly explained. iopl() is a
straightforward way to allow direct access to all I/O ports from
user mode, when it is passed the argument 3. In practice, it is
always passed the argument 3.
 
The description ends with the following paragraph:
"This call is deprecated, is significantly slower than ioperm(2),
and is only provided for older X servers which require access to
all 65536 I/O ports.  It is mostly for the i386 architecture.  On
many other architectures it does not exist or will always return
an error."
 
The (dubious) claim that iopl() is slower than ioperm() is
completeley irrelevant, as the most common use case involves
calling iopl() or ioperm() once during program initialization.
 
Compared to iopl(), ioperm() is more restrictive. In the past
NetBSD, FreeBSD, OpenBSD and DragonFly BSD provided analogous
implementations in the form of i386_set_ioperm(). OpenBSD/x86_64
provided amd64_set_ioperm(). They served the same purpose as
ioperm() on Linux/x86{_64}, i.e. controlling access to specific
I/O ports.
However, as far as I can tell, all mentioned BSDs have dropped
that functionality, but NetBSD and OpenBSD retained their version
of iopl(): i386_iopl(), x86_64_iopl() (NetBSD/x86_64) and
amd64_iopl() (OpenBSD/x86_64). FreeBSD and DragonFly BSD set the
IOPL of the relevant process to 3 in the kernel, when the process
successfully opens /dev/io.
In other words, the BSD projects all preferred to drop their
versions of ioperm() in favor of continuing to provide user
programs a way to change the IOPL. It is interesting and perhaps
noteworthy that the Linux project appears to head in the opposite
direction.

Ingo, Thomas, do you have any comments from the OpenBSD and Linux sides?

quoted

As I stated above the purpose of iopl() goes beyond providing
access only to older X servers. It is a mechanism to allow user
programs to directly communicate with hardware through I/O ports.
Whether this functionality is sufficient for modern hardware is
outside the scope of describing iopl().

Thanks for the report.  Do you have any specific wording you'd like to
change?

quoted

Kind regards,
Daniël Hörchner

Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es>