diff -r 414373f007e8 xen/drivers/passthrough/iommu.c
--- a/xen/drivers/passthrough/iommu.c	Mon Feb 02 09:21:04 2009 +0000
+++ b/xen/drivers/passthrough/iommu.c	Tue Feb 03 13:07:53 2009 -0500
@@ -33,6 +33,8 @@
  *   no-pv                      Disable IOMMU for PV domains (default)
  *   force|required             Don't boot unless IOMMU is enabled
  *   passthrough                Bypass VT-d translation for Dom0
+ *   snoop                      Utilize the snoop control for IOMMU (default)
+ *   no-snoop                   Dont utilize the snoop control for IOMMU
  */
 custom_param("iommu", parse_iommu_param);
 int iommu_enabled = 0;
@@ -45,6 +47,7 @@
 {
     char *ss;
     iommu_enabled = 1;
+    iommu_snoop = 1;
 
     do {
         ss = strchr(s, ',');
@@ -62,6 +65,10 @@
             force_iommu = 1;
         else if ( !strcmp(s, "passthrough") )
             iommu_passthrough = 1;
+        else if ( !strcmp(s, "snoop") )
+            iommu_snoop = 1;
+        else if ( !strcmp(s, "no-snoop") )
+            iommu_snoop = 0;
 
         s = ss + 1;
     } while ( ss );
diff -r 414373f007e8 xen/drivers/passthrough/vtd/dmar.c
--- a/xen/drivers/passthrough/vtd/dmar.c	Mon Feb 02 09:21:04 2009 +0000
+++ b/xen/drivers/passthrough/vtd/dmar.c	Tue Feb 03 13:07:53 2009 -0500
@@ -539,13 +539,15 @@
     /* Giving that all devices within guest use same io page table,
      * enable snoop control only if all VT-d engines support it.
      */
-    iommu_snoop = 1;
-    for_each_drhd_unit ( drhd )
+    if ( iommu_snoop )
     {
-        iommu = drhd->iommu;
-        if ( !ecap_snp_ctl(iommu->ecap) ) {
-            iommu_snoop = 0;
-            break;
+        for_each_drhd_unit ( drhd )
+        {
+            iommu = drhd->iommu;
+            if ( !ecap_snp_ctl(iommu->ecap) ) {
+                iommu_snoop = 0;
+                break;
+            }
         }
     }