trace: improve check_tbuf_size()

It didn't consider the case of the incoming size not allowing for the
2*data_size range for t_buf->{prod,cons}

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff -r 55792c6ec720 xen/common/trace.c
--- a/xen/common/trace.c	Wed Jun 30 15:50:33 2010 +0100
+++ b/xen/common/trace.c	Wed Jun 30 16:10:16 2010 +0100
@@ -80,11 +80,19 @@
 
 /**
  * check_tbuf_size - check to make sure that the proposed size will fit
- * in the currently sized struct t_info.
+ * in the currently sized struct t_info and allows prod and cons to
+ * reach double the value without overflow.
  */
-static inline int check_tbuf_size(int size)
+static int check_tbuf_size(u32 pages)
 {
-    return (num_online_cpus() * size + T_INFO_FIRST_OFFSET) > (T_INFO_SIZE / sizeof(uint32_t));
+    struct t_buf dummy;
+    typeof(dummy.prod) size;
+    
+    size = ((typeof(dummy.prod))pages)  * PAGE_SIZE;
+
+    return (size / PAGE_SIZE != pages)
+           || (size + size < size)
+           || (num_online_cpus() * pages + T_INFO_FIRST_OFFSET > T_INFO_SIZE / sizeof(uint32_t));
 }
 
 /**