[PATCH] VT-d: fix caching mode IOTLB flushing

[Jan Beulich <jbeulich@xxxxxxxx>]

While for context cache entry flushing use of did 0 is indeed correct
(after all upon reading the context entry the IOMMU wouldn't know any
domain ID if the entry is not present, and hence a surrogate one needs
to be used), for IOTLB entries the normal domain ID (from the [present]
context entry) gets used. See sub-section "IOTLB" of section "Address
Translation Caches" in the VT-d spec.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Luckily this is supposed to be an issue only when running on emulated
IOMMUs; hardware implementations are expected to have CAP.CM=0.

--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -474,17 +474,10 @@ int vtd_flush_iotlb_reg(struct vtd_iommu
 
     /*
      * In the non-present entry flush case, if hardware doesn't cache
-     * non-present entry we do nothing and if hardware cache non-present
-     * entry, we flush entries of domain 0 (the domain id is used to cache
-     * any non-present entries)
+     * non-present entries we do nothing.
      */
-    if ( flush_non_present_entry )
-    {
-        if ( !cap_caching_mode(iommu->cap) )
-            return 1;
-        else
-            did = 0;
-    }
+    if ( flush_non_present_entry && !cap_caching_mode(iommu->cap) )
+        return 1;
 
     /* use register invalidation */
     switch ( type )
--- a/xen/drivers/passthrough/vtd/qinval.c
+++ b/xen/drivers/passthrough/vtd/qinval.c
@@ -362,17 +362,10 @@ static int __must_check flush_iotlb_qi(s
 
     /*
      * In the non-present entry flush case, if hardware doesn't cache
-     * non-present entry we do nothing and if hardware cache non-present
-     * entry, we flush entries of domain 0 (the domain id is used to cache
-     * any non-present entries)
+     * non-present entries we do nothing.
      */
-    if ( flush_non_present_entry )
-    {
-        if ( !cap_caching_mode(iommu->cap) )
-            return 1;
-        else
-            did = 0;
-    }
+    if ( flush_non_present_entry && !cap_caching_mode(iommu->cap) )
+        return 1;
 
     /* use queued invalidation */
     if (cap_write_drain(iommu->cap))