[PATCH] xenpaging: allow only one xenpaging call per guest

[Olaf Hering <olaf@aepfle.de>]

Make sure only one xenpaging binary is active per domain.
Print info when the host lacks the required features for xenpaging.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

---
 tools/xenpaging/xenpaging.c |   12 +++++++++++-
 xen/arch/x86/mm/mem_event.c |    7 +++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

--- xen-unstable.hg-4.1.22155.orig/tools/xenpaging/xenpaging.c
+++ xen-unstable.hg-4.1.22155/tools/xenpaging/xenpaging.c
@@ -123,7 +123,17 @@ xenpaging_t *xenpaging_init(xc_interface
                              paging->mem_event.ring_page);
     if ( rc != 0 )
     {
-        ERROR("Error initialising shared page");
+        switch ( errno ) {
+            case EBUSY:
+                ERROR("xenpaging is (or was) active on this domain");
+                break;
+            case ENODEV:
+                ERROR("EPT not supported for this guest");
+                break;
+            default:
+                ERROR("Error initialising shared page");
+                break;
+        }
         goto err;
     }
 
--- xen-unstable.hg-4.1.22155.orig/xen/arch/x86/mm/mem_event.c
+++ xen-unstable.hg-4.1.22155/xen/arch/x86/mm/mem_event.c
@@ -226,6 +226,13 @@ int mem_event_domctl(struct domain *d, x
             mfn_t ring_mfn;
             mfn_t shared_mfn;
 
+            /* Only one xenpaging at a time. If xenpaging crashed,
+             * the cache is in an undefined state and so is the guest
+             */
+            rc = -EBUSY;
+            if ( d->mem_event.enabled )
+                break;
+
             /* Currently only EPT is supported */
             rc = -ENODEV;
             if ( !(hap_enabled(d) &&