This is based upon a report from Chris Torek and his initial patch.
From Chris's report:

--------------------
This came up in testing kgdb, using the built-in tests -- turn
on CONFIG_KGDB_TESTS, then

    echo V1 > /sys/module/kgdbts/parameters/kgdbts

-- but it would affect using kgdb if you were debugging and looking
at bad pointers.
--------------------

When we get a copy_{from,to}_user() request and the %asi is set to
something other than ASI_AIUS (which is userspace) then we branch off
to a routine called memcpy_user_stub().  It just does a straight
memcpy since we are copying from kernel to kernel in this case.

The logic was that since source and destination are both kernel
pointers we don't need to have exception checks.

But for what probe_kernel_{read,write}() is trying to do, we have to
have the checks, otherwise things like kgdb bad kernel pointer
accesses don't do the right thing.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This is an implementation of a suggestion made by Chris Torek:
--------------------
Something else I noticed in passing: the EX and EX_LD/EX_ST macros
scattered throughout the various .S files make a fair bit of .fixup
code, all of which does the same thing.  At the cost of one symbol
in copy_in_user.S, you could just have one common two-instruction
retl-and-mov-1 fixup that they all share.
--------------------

The following is with a defconfig build:

   text	   data	    bss	    dec	    hex	filename
3972767	 344024	 584449	4901240	 4ac978	vmlinux.orig
39688877	 344024	 584449	4897360	 4aba50	vmlinux
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

o Renamed files in sparc64 to <name>_64.S when identical
  to sparc32 files.
o iomap.c were equal for sparc32 and sparc64
o adjusted sparc/Makefile now we have only one lib/
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

For the case where the source is not aligned modulo 8
we don't use load-twins to suck the data in and this
kills performance since normal loads allocate in the
L1 cache (unlike load-twin) and thus big memcpys swipe
the entire L1 D-cache.

We need to allocate a register window to implement this
properly, but that actually simplifies a lot of things
as a nice side-effect.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Signed-off-by: NDavid S. Miller <davem@davemloft.net>