xen: Add support for preemption

Add Xen support for preemption.  This is mostly a cleanup of existing
preempt_enable/disable calls, or just comments to explain the current
usage.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
diff --git a/arch/i386/xen/Kconfig b/arch/i386/xen/Kconfig
index b7697ff..9df99e1 100644
--- a/arch/i386/xen/Kconfig
+++ b/arch/i386/xen/Kconfig
@@ -4,7 +4,7 @@
 
 config XEN
 	bool "Enable support for Xen hypervisor"
-	depends on PARAVIRT && X86_CMPXCHG && X86_TSC && !(PREEMPT || NEED_MULTIPLE_NODES)
+	depends on PARAVIRT && X86_CMPXCHG && X86_TSC && !NEED_MULTIPLE_NODES
 	help
 	  This is the Linux Xen port.  Enabling this will allow the
 	  kernel to boot in a paravirtualized environment under the