} > This approach appears to handle some of the weaknesses of the RTLinux
} > method; namely that a binary module can still contain the cli/sti
} > instructions and destroy the determinism. In fact, even a user mode
} > program with root permissions can do the same. The best example is
} > some of the X servers.
}
} I hadn't thought of this particularly, but you are correct by saying
} that this is a shortcoming of the RTLinux method.
I think this is a combination of weaknesses, not all of which are
characteristics of the RTLinux approach.
X, running as root or not, cannot have a cli/sti in them unless X is
executing in kernel mode (not user mode), which it does not do. Old
versions of X did use ioperm, and linux/x86 allowed that. I think that's a
weakness of the x86, not the RTLinux approach itself. Alpha, MIPS and
PowerPC RTLinux approaches don't suffer from this problem.
Current versions of X (4.x) are pretty aggressive with cards that are not
entirely spec which results in PCI stalls. That's a problem for any
real-time system since software can't do much about this once it's
happened.
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