VT-d: introduce helper to convert DID to domid_t

This is in preparation of adding another "translation" method. Take the
combination of the extra validation both previously open-coded have been
doing: Bounds check and bitmap check. But don't propagate the previous
pointless check of whether ->domid_map[] was actually allocated, as
failure there would lead to overall failure of IOMMU initialization
anyway.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>

xen/drivers/passthrough/vtd/extern.h

@@ -45,6 +45,8 @@ void disable_intremap(struct vtd_iommu *iommu);
 int iommu_alloc(struct acpi_drhd_unit *drhd);
 void iommu_free(struct acpi_drhd_unit *drhd);
 
+domid_t did_to_domain_id(const struct vtd_iommu *iommu, unsigned int did);
+
 int iommu_flush_iec_global(struct vtd_iommu *iommu);
 int iommu_flush_iec_index(struct vtd_iommu *iommu, u8 im, u16 iidx);
 void clear_fault_bits(struct vtd_iommu *iommu);

xen/drivers/passthrough/vtd/iommu.c

@@ -123,15 +123,16 @@ static int context_get_domain_id(const struct context_entry *context,
 
     if ( iommu && context )
     {
-        unsigned int nr_dom = cap_ndoms(iommu->cap);
         unsigned int dom_index = context_domain_id(*context);
 
-        if ( dom_index < nr_dom && iommu->domid_map )
-            domid = iommu->domid_map[dom_index];
-        else
+        domid = did_to_domain_id(iommu, dom_index);
+        if ( domid == DOMID_INVALID )
+        {
             dprintk(XENLOG_DEBUG VTDPREFIX,
-                    "dom_index %u exceeds nr_dom %u or iommu has no domid_map\n",
-                    dom_index, nr_dom);
+                    "no domid for did %u (nr_dom %u)\n",
+                    dom_index, cap_ndoms(iommu->cap));
+            domid = -1;
+        }
     }
 
     return domid;
@@ -193,6 +194,14 @@ static void check_cleanup_domid_map(struct domain *d,
     }
 }
 
+domid_t did_to_domain_id(const struct vtd_iommu *iommu, unsigned int did)
+{
+    if ( did >= cap_ndoms(iommu->cap) || !test_bit(did, iommu->domid_bitmap) )
+        return DOMID_INVALID;
+
+    return iommu->domid_map[did];
+}
+
 static void sync_cache(const void *addr, unsigned int size)
 {
     static unsigned long clflush_size = 0;

xen/drivers/passthrough/vtd/qinval.c

@@ -229,10 +229,7 @@ static int __must_check dev_invalidate_sync(struct vtd_iommu *iommu,
     rc = queue_invalidate_wait(iommu, 0, 1, 1, 1);
     if ( rc == -ETIMEDOUT )
     {
-        struct domain *d = NULL;
-
-        if ( test_bit(did, iommu->domid_bitmap) )
-            d = rcu_lock_domain_by_id(iommu->domid_map[did]);
+        struct domain *d = rcu_lock_domain_by_id(did_to_domain_id(iommu, did));
 
         /*
          * In case the domain has been freed or the IOMMU domid bitmap is