iommu/vt-d: Add for_each_device_domain() helper

This adds a helper named for_each_device_domain() to iterate
over the elements in device_domain_list and invoke a callback
against each element. This allows to search the device_domain
list in other source files.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel-iommu.c

@@ -448,6 +448,27 @@ EXPORT_SYMBOL_GPL(intel_iommu_gfx_mapped);
 static DEFINE_SPINLOCK(device_domain_lock);
 static LIST_HEAD(device_domain_list);
 
+/*
+ * Iterate over elements in device_domain_list and call the specified
+ * callback @fn against each element. This helper should only be used
+ * in the context where the device_domain_lock has already been holden.
+ */
+int for_each_device_domain(int (*fn)(struct device_domain_info *info,
+				     void *data), void *data)
+{
+	int ret = 0;
+	struct device_domain_info *info;
+
+	assert_spin_locked(&device_domain_lock);
+	list_for_each_entry(info, &device_domain_list, global) {
+		ret = fn(info, data);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
 const struct iommu_ops intel_iommu_ops;
 
 static bool translation_pre_enabled(struct intel_iommu *iommu)