xen: don't bother trying to set cr4

Xen ignores all updates to cr4, and some versions will kill the domain if
you try to change its value.  Just ignore all changes.
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/i386/xen/enlighten.c

@@ -623,8 +623,8 @@ static unsigned long xen_read_cr2_direct(void)
 
 static void xen_write_cr4(unsigned long cr4)
 {
-	/* never allow TSC to be disabled */
-	native_write_cr4(cr4 & ~X86_CR4_TSD);
+	/* Just ignore cr4 changes; Xen doesn't allow us to do
+	   anything anyway. */
 }
 
 static unsigned long xen_read_cr3(void)