This causes all non-NMI, non-double-fault kernel entries from
userspace to run on the normal kernel stack.  Double-fault is
exempt to minimize confusion if we double-fault directly from
userspace due to a bad kernel stack.

This is, suprisingly, simpler and shorter than the current code.  It
removes the IMO rather frightening paranoid_userspace path, and it
make sync_regs much simpler.

There is no risk of stack overflow due to this change -- the kernel
stack that we switch to is empty.

This will also enable us to create non-atomic sections within
machine checks from userspace, which will simplify memory failure
handling.  It will also allow the upcoming fsgsbase code to be
simplified, because it doesn't need to worry about usergs when
scheduling in paranoid_exit, as that code no longer exists.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Tony Luck <tony.luck@intel.com>
Acked-by: NBorislav Petkov <bp@alien8.de>
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>

While at it, also refer to the 32 bit entry file.
Signed-off-by: NLuis R. Rodriguez <mcgrof@suse.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: linux-doc@vger.kernel.org
Cc: bpoirier@suse.de
Link: http://lkml.kernel.org/r/1418165684-6226-1-git-send-email-mcgrof@do-not-panic.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

Commit 3ae36655 (x86-64: Rework vsyscall
emulation and add vsyscall= parameter) removed the special use of the
0xcc IDT vector.

Remove it from Documentation/x86/entry_64.txt also.
Signed-off-by: NCesar Eduardo Barros <cesarb@cesarb.net>
Acked-by: NAndy Lutomirski <luto@MIT.EDU>
Link: http://lkml.kernel.org/r/1313447210-9149-1-git-send-email-cesarb@cesarb.netSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>

Signed-off-by: NAndy Lutomirski <luto@mit.edu>
Cc: Jesper Juhl <jj@chaosbits.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Jan Beulich <JBeulich@novell.com>
Cc: richard -rw- weinberger <richard.weinberger@gmail.com>
Cc: Mikael Pettersson <mikpe@it.uu.se>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Louis Rilling <Louis.Rilling@kerlabs.com>
Cc: Valdis.Kletnieks@vt.edu
Cc: pageexec@freemail.hu
Link: http://lkml.kernel.org/r/fc134867cc550977cc996866129e11a16ba0f9ea.1307292171.git.luto@mit.eduSigned-off-by: NIngo Molnar <mingo@elte.hu>