Ingo Molnar wrote:

quoted

+	prod = intf->in_prod;
+	mb();
+	BUG_ON((prod - cons) > sizeof(intf->in));
    

such mb()'s are typically a sign of "i have no clear idea what SMP 
serialization rules apply here, but something is needed because 
otherwise it breaks" ?

Hm, in this case its because it's sharing the memory with Xen, so
there's a particular ordering protocol.  It needs some comments.

    J