diff -r 7809e0941b38 xen/arch/x86/e820.c
--- a/xen/arch/x86/e820.c	Sat Apr 25 08:19:14 2009 +0100
+++ b/xen/arch/x86/e820.c	Tue Apr 28 13:51:20 2009 +0100
@@ -15,6 +15,10 @@
 static void parse_mem(char *s) { opt_mem = parse_size_and_unit(s, NULL); }
 custom_param("mem", parse_mem);
 
+/* opt_nomtrr_check: Don't clip ram to highest cacheable MTRR. */
+static int __initdata opt_no_mtrr_check = 0;
+boolean_param("nomtrr", opt_no_mtrr_check);
+
 struct e820map e820;
 
 static void __init add_memory_region(unsigned long long start,
@@ -444,9 +448,12 @@
 
     reserve_dmi_region();
 
-    top_of_ram = mtrr_top_of_ram();
-    if ( top_of_ram )
-        clip_to_limit(top_of_ram, "MTRRs do not cover all of memory.");
+    if ( !opt_no_mtrr_check )
+    {
+        top_of_ram = mtrr_top_of_ram();
+        if ( top_of_ram )
+            clip_to_limit(top_of_ram, "MTRRs do not cover all of memory.");
+    }
 }
 
 int __init e820_change_range_type(