[RFC] MM: stub domains and page ownership for mapping in dom0

[Samuel Thibault <samuel.thibault@eu.citrix.com>]

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?

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>

diff -r fb74347d80c3 xen/arch/x86/mm.c
--- a/xen/arch/x86/mm.c	Wed Mar 05 12:20:40 2008 +0000
+++ b/xen/arch/x86/mm.c	Thu Mar 06 18:35:55 2008 +0000
@@ -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