on intel processors, a general protection exception is fired if a non-canonical address is loaded into PC during SYSRET. this will cause the kernel to panic.
see http://www.kb.cert.org/vuls/id/649219 and the intel software developer manual for more information.
kmapindex has to be per process, not per mach, as the process
can be switched to another processor while the mapping is
established.
to bootstrap the first process, we have to temporarily set up
so the kmap MMU's can be attached to the process. previously
we assumed that the first two pages for the initial process
where below 2GB and could be accessed with KADDR() directly.
with 16GB machine, all the 2GB above KZERO are dedicated to
the kernel so the user pages returned by newpage() need to
be mapped.
no need for switch here, just calculate the values. also fixes
6l warning about uninitialized min/max (compiler assumes none
of the case statements could match).
this doubling affects all segment types, not just bss.
(tho text/data are usually small...)
and theres no telling if the segment will actually
grow in the future justifying the reduction of memmove
overhead in ibrk().
some ape programs are approaching the 16mb ssegmap size
so that code might trigger.
removing the smarts...
we have to keep kmap page tables in ther own list
because user tables are subject to (virtual) tlb flushing.
we never free kmap page tables except in mmurelease()
where we just link the kmap mmu list in front of the user
mmus and call mmufree() which will free all the mmu's
of the process.
as erik quanstro suggests, theres not much of a point in
storing the full 64bit pc as one cannot get a code segment
bigger than 4G and amd64 makes it hard to use a pc that
isnt 64bit sign extension of 32bit.
instead, we only store ulong (as originally), but sign
extend back when returning in getmalloctag() and
getrealloctag().
getrealloctag() used to be broken. its now fixed.
