I like the idea in theory.
At the same time it just makes me cringe horribly from a security
standpoint. Imagine somebody changing that local mapping to use a trojan
horse system call, and then executing a setuid binary. Ugh.
Also, it would make it impossible to use the global TLB trick, but that
may not be worth worrying about.
So I think it's a great notion, but it just sucks in real life. And it
wouldn't allow the interesting things like gettimeofday() that really only
the kernel can do: you can calibrate the clock in user space, but you
cannot do things like automatically re-calibrate after a suspend event
etc, which is critical on laptops and will be even more so with the new
dual-frequency coppermine CPU's from Intel... (were not only the base time
changes, but the frequency of the clock changes too).
There _are_ a lot of issues like this where the kernel really knows things
that user space can not easily know.
Linus
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