these are the changes to the plan 9 kernel to
add per process segment descriptors.

the 9/ directory contains all the changed files from
/sys/src/9. kernel.diff contains the changes.

the changes in detail are:

pc/fns.h
1) prototype for lldt() added (for l.s)

2) prototype for procfork() added

3) changed userureg macro to test for the rpl bits but
not for the specific UDSEL selector. this is because
with the patch, a process is allowed to change its
data segment. so we test only for the "requested
priviledge level" bits.

pc/dat.h
1) moved the Segdesc struct declaration.

2) added ldt and gdt to PMMU struct

pc/mem.h
1) added segment definitions for LDT and PROCSEGs

pc/io.h
1) added vector constants for general protection
fault and seg not present.

pc/trap.c
1) replaced the user test with userureg() macro

2) added GPF handler to trap() that catches the
fault when we return to userspace, but the segment
registers contain invalid selectors. previously this
was not needed as the segment registers for user
space where constant and checked.

3) in the case we have a trap in kernel mode, the
processor doesnt push the SS and SP registers.
fixed up dumpregs() to print the right values.

4) set default segments in notify()

5) removed the segment check in notify as we
handle it with the GPF handler in trap(). only
make sure the user doesnt change the code
and stack segments to kernel ones.

6) set default segments in execregs()

7) changing of segments from devproc is ok
now as long as code and stack don't point to
kernel segments. changed in setregisters()

pc/mmu.c
1) added switching of procsegs and ldt in mmuswitch()

2) free the ldt in mmurelease()

pc/l.s
1) added lldt() function as it is requred for
switching the ldt.

2) added load_fs() and load_gs() as it is needed
in the GPF handler.

pc/segdesc.c
1) this file contains the code to change the per
process descriptors. it adds the ldt and gdt files
to devarch.

pc/main.c
1) setup gdt and ldt in procsetup()

2) added procfork() to copy ldt and gdt when forking

*/main.c
1) added empty procfork() as well

*/fns.h
1) added procfork() prototype

port/sysproc.c
1) added call to forkfork() from sysfork() to the ldt and gdt
get copied