xen64: deal with extra words Xen pushes onto exception frames

Xen pushes two extra words containing the values of rcx and r11.  This
pvop hook copies the words back into their appropriate registers, and
cleans them off the stack.  This leaves the stack in native form, so
the normal handler can run unchanged.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -1091,7 +1091,7 @@ static const struct pv_irq_ops xen_irq_ops __initdata = {
 	.safe_halt = xen_safe_halt,
 	.halt = xen_halt,
 #ifdef CONFIG_X86_64
-	.adjust_exception_frame = paravirt_nop,
+	.adjust_exception_frame = xen_adjust_exception_frame,
 #endif
 };
 

arch/x86/xen/xen-asm_64.S

@@ -133,6 +133,11 @@ check_events:
 	ret
 #endif
 
+ENTRY(xen_adjust_exception_frame)
+	mov 8+0(%rsp),%rcx
+	mov 8+8(%rsp),%r11
+	ret $16
+
 ENTRY(xen_iret)
 	pushq $0
 	jmp hypercall_page + __HYPERVISOR_iret * 32

arch/x86/xen/xen-ops.h

@@ -67,7 +67,9 @@ DECL_ASM(void, xen_irq_disable_direct, void);
 DECL_ASM(unsigned long, xen_save_fl_direct, void);
 DECL_ASM(void, xen_restore_fl_direct, unsigned long);
 
+/* These are not functions, and cannot be called normally */
 void xen_iret(void);
 void xen_sysexit(void);
+void xen_adjust_exception_frame(void);
 
 #endif /* XEN_OPS_H */