Nothing in princple. In practice it's triggering an amazing variety of
toolchain bugs.
The powerpc guys tried it, and it doesn't work. per-cpu is not
semantically equivalent to per-thread. If you have a function in which
you refer to a percpu variable and then have a preemptable section in
the middle followed by another reference to the same percpu variable,
it's hard to stop gcc from caching a reference to the old tls variable,
even though we may have switched cpus in the meantime.
Also, we explicitly use the other segment register in kernel mode, to
avoid segment register switches where possible. Even with
-mcmodel=kernel, gcc generates %fs references to tls variables.
J
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