diff -r 5dabbf2826c5 xen/drivers/passthrough/iommu.c
--- a/xen/drivers/passthrough/iommu.c	Mon Jan 25 09:58:53 2010 +0800
+++ b/xen/drivers/passthrough/iommu.c	Tue Jan 26 21:17:46 2010 +0800
@@ -30,6 +30,8 @@ static int iommu_populate_page_table(str
  *   pv                         Enable IOMMU for PV domains
  *   no-pv                      Disable IOMMU for PV domains (default)
  *   force|required             Don't boot unless IOMMU is enabled
+ *   workaround_bios_bug        Workaround some bios issue to still enable
+                                VT-d, don't guarantee security
  *   passthrough                Enable VT-d DMA passthrough (no DMA
  *                              translation for Dom0)
  *   no-snoop                   Disable VT-d Snoop Control
@@ -40,6 +42,7 @@ int iommu_enabled = 1;
 int iommu_enabled = 1;
 int iommu_pv_enabled;
 int force_iommu;
+int iommu_workaround_bios_bug;
 int iommu_passthrough;
 int iommu_snoop = 1;
 int iommu_qinval = 1;
@@ -65,6 +68,8 @@ static void __init parse_iommu_param(cha
             iommu_pv_enabled = 0;
         else if ( !strcmp(s, "force") || !strcmp(s, "required") )
             force_iommu = 1;
+        else if ( !strcmp(s, "workaround_bios_bug") )
+            iommu_workaround_bios_bug = 1;
         else if ( !strcmp(s, "passthrough") )
             iommu_passthrough = 1;
         else if ( !strcmp(s, "no-snoop") )
diff -r 5dabbf2826c5 xen/drivers/passthrough/vtd/dmar.c
--- a/xen/drivers/passthrough/vtd/dmar.c	Mon Jan 25 09:58:53 2010 +0800
+++ b/xen/drivers/passthrough/vtd/dmar.c	Tue Jan 26 21:16:49 2010 +0800
@@ -421,17 +421,21 @@ acpi_parse_one_drhd(struct acpi_dmar_ent
         if ( invalid_cnt )
         {
             xfree(dmaru);
-            if ( invalid_cnt == dmaru->scope.devices_cnt )
+
+            if ( iommu_workaround_bios_bug &&
+                 invalid_cnt == dmaru->scope.devices_cnt )
             {
                 dprintk(XENLOG_WARNING VTDPREFIX,
-                    "  Ignore the DRHD due to all devices under "
-                    "its scope are not PCI discoverable!\n");
+                    "  Workaround BIOS bug: ignore the DRHD due to all "
+                    "devices under its scope are not PCI discoverable!\n");
             }
             else
             {
                 dprintk(XENLOG_WARNING VTDPREFIX,
-                    "  The DRHD is invalid due to some devices under "
-                    "its scope are not PCI discoverable!\n");
+                    "  The DRHD is invalid due to there are devices under "
+                    "its scope are not PCI discoverable! Pls try option "
+                    "iommu=force or iommu=workaround_bios_bug if you "
+                    "really want VT-d\n");
                 ret = -EINVAL;
             }
         }
diff -r 5dabbf2826c5 xen/include/xen/iommu.h
--- a/xen/include/xen/iommu.h	Mon Jan 25 09:58:53 2010 +0800
+++ b/xen/include/xen/iommu.h	Tue Jan 26 21:17:08 2010 +0800
@@ -29,6 +29,7 @@ extern int iommu_enabled;
 extern int iommu_enabled;
 extern int iommu_pv_enabled;
 extern int force_iommu;
+extern int iommu_workaround_bios_bug;
 extern int iommu_passthrough;
 extern int iommu_snoop;
 extern int iommu_qinval;