xen: disable interrupts before saving in percpu

Impact: Fix race condition

xen_mc_batch has a small preempt race where it takes the address of a
percpu variable immediately before disabling interrupts, thereby
leaving a small window in which we may migrate to another cpu and save
the flags in the wrong percpu variable.  Disable interrupts before
saving the old flags in a percpu.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/xen/multicalls.h

@@ -19,8 +19,10 @@ DECLARE_PER_CPU(unsigned long, xen_mc_irq_flags);
    paired with xen_mc_issue() */
 static inline void xen_mc_batch(void)
 {
+	unsigned long flags;
 	/* need to disable interrupts until this entry is complete */
-	local_irq_save(__get_cpu_var(xen_mc_irq_flags));
+	local_irq_save(flags);
+	__get_cpu_var(xen_mc_irq_flags) = flags;
 }
 
 static inline struct multicall_space xen_mc_entry(size_t args)