SMP support for n64 patch.

[M. Warner Losh]

In message: <AANLkTimQVoRTc3CHNGHNAS2-PRqiZ8BiStekp6tj1Qc4 at mail.gmail.com>
            "Jayachandran C." <c.jayachandran at gmail.com> writes:
: I've attached a patch that enables SMP support for N64 builds (I can
: get XLR come up multi-user with 32 cpus with this).
: 
: I ended up making KX bit in status register set when you are in
: userspace, since the PCPU area is now mapped in XKSEG.  The PCPU area
: needs to be accessed in  I have reverted the earlier change that
: enabled KX on kernel entry from userspace since that is no longer
: needed.
: 
: Please let me know your comments.

If I understand correctly, and I'm sure the pedants will tell me
otherwise, the KX bit means that a legal reference to a kernel address
is made, it will be interpreted as a 64-bit address when KX is set,
and a 32-bit address when it is clear.

It does not grant any additional access to the kernel memory that
isn't already granted by the TLB entries, correct?  It doesn't grant
access to the kernel memory, correct?

But I'm a little confused.  I'm reading through section 4.5 of the
Privileged Resources Architecture document from mips
(MD00091-2B-MIPS74PRA-AFP-02.50.pdf) and table 4-1 suggests that
running with it in user mode won't change what can be accessed.  So
why would it need to be turned on at all in user mode?  Rather than
the xtlb routine being called for the refill for a kernel address, it
will fault instead.

Or is this an optimization for the xtlb handler which needs to get to
pcpu data?

Warner