MM: stub domains and page ownership for mapping in dom0

In the case of an ioemu stubdomain with SDL or vnc server running in dom0, we want to have the stubdomain expose the HVM guest's video RAM through PVFB.

However, to map the pages from dom0, xenfb uses xc_map_foreign_pages with the stubdomain ID as domid (since that is what is advertised in the PVFB protocol, and needed for other parts of the protocol), and thus get_page_from_l1e() complains because the stubdomain is not the owner of the pages. In such case, this patch checks that the stubdomain has privileges on the actual owner of the pages, and then accept the mapping.

Any red flag?

diff -r fb74347d80c3 xen/arch/x86/mm.c
@@ -673,6 +673,14 @@ get_page_from_l1e
         return 1;
     }
 
+    /* Let privileged domains transfer the right to map their target domain's
+     * pages */
+    {
+        struct domain *owner = page_get_owner(page);
+        if (d != owner && IS_PRIV_FOR(d, owner))
+            d = owner;
+    }
+
     /* Foreign mappings into guests in shadow external mode don't
      * contribute to writeable mapping refcounts.  (This allows the
      * qemu-dm helper process in dom0 to map the domain's memory without

Really pvfb protocol should be using grants for foreign mappings. Didn't Pat Campbell have a patch for that?

This patch is probably a reasonable stopgap however.

-- Keir