tools/virtiofsd: xattr name mapping examples

Add a few examples of xattrmaps to the documentation.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20201023165812.36028-5-dgilbert@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

docs/tools/virtiofsd.rst

@@ -219,6 +219,56 @@ e.g.:
 
   would hide 'security.' xattr's in listxattr from the server.
 
+xattr-mapping Examples
+----------------------
+
+1) Prefix all attributes with 'user.virtiofs.'
+
+::
+
+-o xattrmap=":prefix:all::user.virtiofs.::bad:all:::"
+
+
+This uses two rules, using : as the field separator;
+the first rule prefixes and strips 'user.virtiofs.',
+the second rule hides any non-prefixed attributes that
+the host set.
+
+2) Prefix 'trusted.' attributes, allow others through
+
+::
+
+   "/prefix/all/trusted./user.virtiofs./
+    /bad/server//trusted./
+    /bad/client/user.virtiofs.//
+    /ok/all///"
+
+
+Here there are four rules, using / as the field
+separator, and also demonstrating that new lines can
+be included between rules.
+The first rule is the prefixing of 'trusted.' and
+stripping of 'user.virtiofs.'.
+The second rule hides unprefixed 'trusted.' attributes
+on the host.
+The third rule stops a guest from explicitly setting
+the 'user.virtiofs.' path directly.
+Finally, the fourth rule lets all remaining attributes
+through.
+
+3) Hide 'security.' attributes, and allow everything else
+
+::
+
+    "/bad/all/security./security./
+     /ok/all///'
+
+The first rule combines what could be separate client and server
+rules into a single 'all' rule, matching 'security.' in either
+client arguments or lists returned from the host.  This stops
+the client seeing any 'security.' attributes on the server and
+stops it setting any.
+
 Examples
 --------