Re: %gs:0 thread pseudoregister in oskit-mach

[Roland McGrath]

> The UTCB is a central information page for L4 userspace threads
> and it's pretty difficult to find another x86 register that can
> be used to point to it.

What's wrong with %fs?  But anyway, that is fine if the one word at %gs:0
can be made available for user use.  Ideally that word would be writable
(which you could manage along with read-only use by situating the segment
base on a page boundary, one page writable and one not).  But the only
thing that would write it is pthreads innards, so it would be workable to
have a system call to set the %gs:0 word for a thread.

I don't know the L4 interfaces, but why not just make this page accessible
at a canonical address accessible from the standard user %ds segment?