Re: iopl

H. Peter Anvin (hpa@transmeta.com)
21 Sep 1998 16:49:04 GMT

Followup to: <199809052027.WAA01372@manjak.knm.org.pl>
By author: Guest <guest@manjak.knm.org.pl>
In newsgroup: linux.dev.kernel
>
> Hello,
>
> I've got a question concerning iopl() syscall. What are
> the parameters of the function... I mean which level
> give me what priviledges ?! I've looked at the
> kernel sources, but those bitmasks doesn't say me anything.
> ... so what gives me:
>
> iopl(1)
> iopl(2)
> iopl(3)
>
> Thanks,
> Martin
>

iopl(1) or iopl(2) don't buy you anything; iopl(3) lets you do any I/O.

Background: the x86 considers a process "I/O privileged" if
IOPL >= CPL. Since a user-space process in Linux has CPL=3 and a
kernel-space process has CPL=0, only IOPL=0 and IOPL=3 really matter.

-hpa

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