Clarify topology information for volume and snapshot

Author: xing-yang

csi.proto

@@ -243,6 +243,11 @@ message CreateVolumeRequest {
   // If this field is not specified and the SP has the
   // VOLUME_ACCESSIBILITY_CONSTRAINTS plugin capability, the SP MAY
   // choose where the provisioned volume is accessible from.
+  // When creating a volume from a snapshot, SP SHALL look at
+  // accessibility_requirements in CreateVolumeRequest and
+  // accessible_topology in CreateSnapshotResponse, and find an
+  // intersection of the two. This intersection SHALL be reflected in
+  // accessible_topology of CreateVolumeResponse.
   TopologyRequirement accessibility_requirements = 7;
 }
 
@@ -1183,9 +1188,12 @@ message NodeGetInfoResponse {
   // VOLUME_ACCESSIBILITY_CONSTRAINTS or
   // SNAPSHOT_ACCESSIBILITY_CONSTRAINTS plugin capability.
   // COs MAY use this information along with the topology information
-  // returned in CreateVolumeResponse CreateSnapshotResponse to ensure
-  // that a given volume or snapshot is accessible from a given node
-  // when scheduling workloads.
+  // returned in CreateVolumeResponse or CreateSnapshotResponse to
+  // ensure that a given volume or snapshot is accessible from a given
+  // node when scheduling workloads.
+  // For creating a volume from a snapshot, the topology information in
+  // CreateVolumeResponse is the intersection of topologies accessible
+  // by the volume and its source snapshot.
   // This field is OPTIONAL. If it is not specified, the CO MAY assume
   // the node is not subject to any topological constraint, and MAY
   // schedule workloads that reference any volume V, such that there are