[PATCH 1/6] VT-d: properly reserve DID 0 for caching mode IOMMUs

[Jan Beulich]

Merely setting bit 0 in the bitmap is insufficient, as then Dom0 will
still have DID 0 allocated to it, because of the zero-filling of
domid_map[]. Set slot 0 to DOMID_INVALID to keep DID 0 from getting
used.

Fixes: b9c20c78789f ("VT-d: per-iommu domain-id")
Signed-off-by: Jan Beulich <jbeul...@suse.com>

--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -1257,16 +1257,19 @@ int __init iommu_alloc(struct acpi_drhd_
     if ( !iommu->domid_bitmap )
         return -ENOMEM;
 
+    iommu->domid_map = xzalloc_array(u16, nr_dom);
+    if ( !iommu->domid_map )
+        return -ENOMEM;
+
     /*
-     * if Caching mode is set, then invalid translations are tagged with
-     * domain id 0, Hence reserve bit 0 for it
+     * If Caching mode is set, then invalid translations are tagged with
+     * domain id 0. Hence reserve bit/slot 0.
      */
     if ( cap_caching_mode(iommu->cap) )
+    {
+        iommu->domid_map[0] = DOMID_INVALID;
         __set_bit(0, iommu->domid_bitmap);
-
-    iommu->domid_map = xzalloc_array(u16, nr_dom);
-    if ( !iommu->domid_map )
-        return -ENOMEM;
+    }
 
     return 0;
 }