x86/iommu: add comment regarding setting of need_sync

Clarify why relaxed hardware domains don't need iommu page-table
syncing.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/drivers/passthrough/iommu.c b/xen/drivers/passthrough/iommu.c
index 79ec6719f5722179f1258b8385c207b284429b70..f8c3bf53bd1793df93d7ddea6564dc929f40c156 100644 (file)
--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -185,6 +185,10 @@ void __hwdom_init iommu_hwdom_init(struct domain *d)
     register_keyhandler('o', &iommu_dump_p2m_table, "dump iommu p2m table", 0);
 
     hd->status = IOMMU_STATUS_initializing;
+    /*
+     * NB: relaxed hw domains don't need sync because all ram is already
+     * mapped in the iommu page tables.
+     */
     hd->need_sync = iommu_hwdom_strict && !iommu_use_hap_pt(d);
     if ( need_iommu_pt_sync(d) )
     {