Add upstream fix for 4.15

This is from xen-devel and isn't yet in master.

Without it PV is broken.

patches.xen/x86-xen-init-gs-very-early-to-avoid-page-faults-with.patch

@@ -0,0 +1,56 @@
+From d3c8038919ffb95553ff2b6878f6eae46579abe7 Mon Sep 17 00:00:00 2001
+From: Juergen Gross <jgross@suse.com>
+Date: Thu, 1 Feb 2018 13:40:19 +0100
+Subject: [PATCH] x86/xen: init %gs very early to avoid page faults with stack
+ protector
+
+When running as Xen pv guest %gs is initialized some time after
+C code is started. Depending on stack protector usage this might be
+too late, resulting in page faults.
+
+So setup %gs and MSR_GS_BASE in assembly code already.
+
+Cc: stable@vger.kernel.org
+Signed-off-by: Juergen Gross <jgross@suse.com>
+---
+ arch/x86/xen/xen-head.S | 16 ++++++++++++++++
+ 1 file changed, 16 insertions(+)
+
+diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
+index 497cc55a0c16..96f26e026783 100644
+--- a/arch/x86/xen/xen-head.S
++++ b/arch/x86/xen/xen-head.S
+@@ -9,7 +9,9 @@
+ 
+ #include <asm/boot.h>
+ #include <asm/asm.h>
++#include <asm/msr.h>
+ #include <asm/page_types.h>
++#include <asm/percpu.h>
+ #include <asm/unwind_hints.h>
+ 
+ #include <xen/interface/elfnote.h>
+@@ -35,6 +37,20 @@ ENTRY(startup_xen)
+ 	mov %_ASM_SI, xen_start_info
+ 	mov $init_thread_union+THREAD_SIZE, %_ASM_SP
+ 
++#ifdef CONFIG_X86_64
++	/* Set up %gs.
++	 *
++	 * The base of %gs always points to the bottom of the irqstack
++	 * union.  If the stack protector canary is enabled, it is
++	 * located at %gs:40.  Note that, on SMP, the boot cpu uses
++	 * init data section till per cpu areas are set up.
++	 */
++	movl	$MSR_GS_BASE,%ecx
++	movq	$INIT_PER_CPU_VAR(irq_stack_union),%rax
++	cdq
++	wrmsr
++#endif
++
+ 	jmp xen_start_kernel
+ END(startup_xen)
+ 	__FINIT
+-- 
+2.15.1
+

series.conf

@@ -34,6 +34,9 @@ patches.xen/Revert-xen-avoid-deadlock-in-xenbus-driver.patch
 patches.xen/Revert-xenbus-remove-transaction-holder-from-list-be.patch
 patches.xen/Revert-xen-optimize-xenbus-driver-for-multiple-concu.patch
 
+# Upstream patch for 4.15
+patches.xen/x86-xen-init-gs-very-early-to-avoid-page-faults-with.patch
+
 # Hardware-specific fixes, backported from newer kernel and/or developers branches
 patches.backports/v2-2-2-Input-synaptics---Lenovo-X1-Carbon-5-should-use-SMBUS-RMI.patch
 patches.backports/Input-synaptics---add-Intertouch-support-on-X1-Carbon-6th-and-X280.patch