Add the rt linux 4.1.3-rt3 as base

Import the rt linux 4.1.3-rt3 as OPNFV kvm base.

It's from git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-4.1.y-rt and
the base is:

commit 0917f823c59692d751951bf5ea699a2d1e2f26a2
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Sat Jul 25 12:13:34 2015 +0200

    Prepare v4.1.3-rt3

    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

We lose all the git history this way and it's not good. We
should apply another opnfv project repo in future.

Change-Id: I87543d81c9df70d99c5001fbdf646b202c19f423
Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>

1 files changed, 104 insertions, 0 deletions

diff --git a/kernel/Documentation/x86/entry_64.txt b/kernel/Documentation/x86/entry_64.txt
new file mode 100644
index 000000000..9132b8617
--- /dev/null
+++ b/kernel/Documentation/x86/entry_64.txt
@@ -0,0 +1,104 @@
+This file documents some of the kernel entries in
+arch/x86/kernel/entry_64.S.  A lot of this explanation is adapted from
+an email from Ingo Molnar:
+
+http://lkml.kernel.org/r/<20110529191055.GC9835%40elte.hu>
+
+The x86 architecture has quite a few different ways to jump into
+kernel code.  Most of these entry points are registered in
+arch/x86/kernel/traps.c and implemented in arch/x86/kernel/entry_64.S
+for 64-bit, arch/x86/kernel/entry_32.S for 32-bit and finally
+arch/x86/ia32/ia32entry.S which implements the 32-bit compatibility
+syscall entry points and thus provides for 32-bit processes the
+ability to execute syscalls when running on 64-bit kernels.
+
+The IDT vector assignments are listed in arch/x86/include/asm/irq_vectors.h.
+
+Some of these entries are:
+
+ - system_call: syscall instruction from 64-bit code.
+
+ - ia32_syscall: int 0x80 from 32-bit or 64-bit code; compat syscall
+   either way.
+
+ - ia32_syscall, ia32_sysenter: syscall and sysenter from 32-bit
+   code
+
+ - interrupt: An array of entries.  Every IDT vector that doesn't
+   explicitly point somewhere else gets set to the corresponding
+   value in interrupts.  These point to a whole array of
+   magically-generated functions that make their way to do_IRQ with
+   the interrupt number as a parameter.
+
+ - APIC interrupts: Various special-purpose interrupts for things
+   like TLB shootdown.
+
+ - Architecturally-defined exceptions like divide_error.
+
+There are a few complexities here.  The different x86-64 entries
+have different calling conventions.  The syscall and sysenter
+instructions have their own peculiar calling conventions.  Some of
+the IDT entries push an error code onto the stack; others don't.
+IDT entries using the IST alternative stack mechanism need their own
+magic to get the stack frames right.  (You can find some
+documentation in the AMD APM, Volume 2, Chapter 8 and the Intel SDM,
+Volume 3, Chapter 6.)
+
+Dealing with the swapgs instruction is especially tricky.  Swapgs
+toggles whether gs is the kernel gs or the user gs.  The swapgs
+instruction is rather fragile: it must nest perfectly and only in
+single depth, it should only be used if entering from user mode to
+kernel mode and then when returning to user-space, and precisely
+so. If we mess that up even slightly, we crash.
+
+So when we have a secondary entry, already in kernel mode, we *must
+not* use SWAPGS blindly - nor must we forget doing a SWAPGS when it's
+not switched/swapped yet.
+
+Now, there's a secondary complication: there's a cheap way to test
+which mode the CPU is in and an expensive way.
+
+The cheap way is to pick this info off the entry frame on the kernel
+stack, from the CS of the ptregs area of the kernel stack:
+
+	xorl %ebx,%ebx
+	testl $3,CS+8(%rsp)
+	je error_kernelspace
+	SWAPGS
+
+The expensive (paranoid) way is to read back the MSR_GS_BASE value
+(which is what SWAPGS modifies):
+
+	movl $1,%ebx
+	movl $MSR_GS_BASE,%ecx
+	rdmsr
+	testl %edx,%edx
+	js 1f   /* negative -> in kernel */
+	SWAPGS
+	xorl %ebx,%ebx
+1:	ret
+
+If we are at an interrupt or user-trap/gate-alike boundary then we can
+use the faster check: the stack will be a reliable indicator of
+whether SWAPGS was already done: if we see that we are a secondary
+entry interrupting kernel mode execution, then we know that the GS
+base has already been switched. If it says that we interrupted
+user-space execution then we must do the SWAPGS.
+
+But if we are in an NMI/MCE/DEBUG/whatever super-atomic entry context,
+which might have triggered right after a normal entry wrote CS to the
+stack but before we executed SWAPGS, then the only safe way to check
+for GS is the slower method: the RDMSR.
+
+Therefore, super-atomic entries (except NMI, which is handled separately)
+must use idtentry with paranoid=1 to handle gsbase correctly.  This
+triggers three main behavior changes:
+
+ - Interrupt entry will use the slower gsbase check.
+ - Interrupt entry from user mode will switch off the IST stack.
+ - Interrupt exit to kernel mode will not attempt to reschedule.
+
+We try to only use IST entries and the paranoid entry code for vectors
+that absolutely need the more expensive check for the GS base - and we
+generate all 'normal' entry points with the regular (faster) paranoid=0
+variant.