[parisc-linux] Lightweight syscalls

[Matthew Wilcox]

I've just written a `lightweight syscall' which will be used to set cr27
to a per-thread value.  Comments, criticism, etc all welcome.

set_thread_pointer:
        gate    .+8, %r0                /* increase privilege */
        depi    3, 31, 2, %r31          /* Ensure we return into user mode. */
        be      0(%sr3,%r31)            /* return to user space */
        mtctl   %r26, %cr27             /* move arg0 to the control register */

note that the intended model is to place this at a fixed location on the
gateway page, and it will be called with BLE.  I haven't tested this yet,
in the best traditions of linux :-)

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