x86: prevent PGE flush from interruption/preemption

CR4 manipulation is not protected against interrupts and preemption,
but KVM uses smp_function_call to manipulate the X86_CR4_VMXE bit
either from the CPU hotplug code or from the kvm_init call.

We need to protect the CR4 manipulation from both interrupts and
preemption.

Original bug report: http://lkml.org/lkml/2008/5/7/48
Bugzilla entry: http://bugzilla.kernel.org/show_bug.cgi?id=10642

This is not a regression from 2.6.25, it's a long standing and hard to
trigger bug.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/asm-x86/tlbflush.h

@@ -22,12 +22,23 @@ static inline void __native_flush_tlb(void)
 
 static inline void __native_flush_tlb_global(void)
 {
-	unsigned long cr4 = read_cr4();
+	unsigned long flags;
+	unsigned long cr4;
 
+	/*
+	 * Read-modify-write to CR4 - protect it from preemption and
+	 * from interrupts. (Use the raw variant because this code can
+	 * be called from deep inside debugging code.)
+	 */
+	raw_local_irq_save(flags);
+
+	cr4 = read_cr4();
 	/* clear PGE */
 	write_cr4(cr4 & ~X86_CR4_PGE);
 	/* write old PGE again and flush TLBs */
 	write_cr4(cr4);
+
+	raw_local_irq_restore(flags);
 }
 
 static inline void __native_flush_tlb_single(unsigned long addr)