commit 272793dcb989fc1ff2caaa9519f8f1ea5434b578
Author: Juergen Gross <jgross@xxxxxxxx>
Date:   Wed May 11 07:53:54 2016 +0200

    xen: register early page fault handler
    
    In early boot of dom0 accesses to the sparse m2p list of the hypervisor
    can result in unhandled page faults as the #PF handler handling this
    case via exception table isn't yet registered.
    
    Install a primitive early page fault handler for this case.

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 858b555..a20ea98 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -911,6 +911,7 @@ idtentry stack_segment		do_stack_segment	has_error_code=1
 idtentry xen_debug		do_debug		has_error_code=0
 idtentry xen_int3		do_int3			has_error_code=0
 idtentry xen_stack_segment	do_stack_segment	has_error_code=1
+idtentry xen_page_fault		xen_do_page_fault	has_error_code=1
 #endif
 
 idtentry general_protection	do_general_protection	has_error_code=1
diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index c3496619..f91cb3f 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -16,6 +16,7 @@ asmlinkage void int3(void);
 asmlinkage void xen_debug(void);
 asmlinkage void xen_int3(void);
 asmlinkage void xen_stack_segment(void);
+asmlinkage void xen_page_fault(void);
 asmlinkage void overflow(void);
 asmlinkage void bounds(void);
 asmlinkage void invalid_op(void);
@@ -54,6 +55,7 @@ asmlinkage void trace_page_fault(void);
 #define trace_alignment_check alignment_check
 #define trace_simd_coprocessor_error simd_coprocessor_error
 #define trace_async_page_fault async_page_fault
+#define trace_xen_page_fault xen_page_fault
 #endif
 
 dotraplinkage void do_divide_error(struct pt_regs *, long);
@@ -74,6 +76,7 @@ asmlinkage struct pt_regs *sync_regs(struct pt_regs *);
 #endif
 dotraplinkage void do_general_protection(struct pt_regs *, long);
 dotraplinkage void do_page_fault(struct pt_regs *, unsigned long);
+dotraplinkage void xen_do_page_fault(struct pt_regs *, unsigned long);
 #ifdef CONFIG_TRACING
 dotraplinkage void trace_do_page_fault(struct pt_regs *, unsigned long);
 #else
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 7ab2951..eaee9d3 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -17,7 +17,10 @@
 #include <asm/e820.h>
 #include <asm/setup.h>
 #include <asm/acpi.h>
+#include <asm/desc.h>
 #include <asm/numa.h>
+#include <asm/traps.h>
+#include <asm/uaccess.h>
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>
 
@@ -1067,4 +1070,19 @@ void __init xen_arch_setup(void)
 #ifdef CONFIG_NUMA
 	numa_off = 1;
 #endif
+
+	sort_main_extable();
+	set_intr_gate(X86_TRAP_PF, xen_page_fault);
+}
+
+/*
+ * Early page fault handler being capable to handle page faults resulting
+ * from accesses via xen_safe_read_ulong().
+ * This page fault handler will be active in early boot only. It is being
+ * replaced by the default page fault handler later.
+ */
+dotraplinkage void notrace
+xen_do_page_fault(struct pt_regs *regs, unsigned long error_code)
+{
+	fixup_exception(regs, X86_TRAP_PF);
 }