xentrace: correct formula to calculate t_info_pages

(xen-unstable changeset 23128:4a335f1000ea)

The current formula to calculate t_info_pages, based on the initial
code, is slightly incorrect. It may allocate more than needed.
Each cpu has some pages/mfns stored as uint32_t.
That list is stored with an offset at tinfo. 

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

---
 xen/common/trace.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

Index: xen-4.1.1-testing/xen/common/trace.c
===================================================================
--- xen-4.1.1-testing.orig/xen/common/trace.c
+++ xen-4.1.1-testing/xen/common/trace.c
@@ -111,7 +111,7 @@ static int calculate_tbuf_size(unsigned
 {
     struct t_buf dummy;
     typeof(dummy.prod) size;
-    unsigned int t_info_words, t_info_bytes;
+    unsigned int t_info_words;
 
     /* force maximum value for an unsigned type */
     size = -1;
@@ -126,9 +126,8 @@ static int calculate_tbuf_size(unsigned
         pages = size;
     }
 
-    t_info_words = num_online_cpus() * pages + t_info_first_offset;
-    t_info_bytes = t_info_words * sizeof(uint32_t);
-    t_info_pages = PFN_UP(t_info_bytes);
+    t_info_words = num_online_cpus() * pages * sizeof(uint32_t);
+    t_info_pages = PFN_UP(t_info_first_offset + t_info_words);
     printk(XENLOG_INFO "xentrace: requesting %u t_info pages "
            "for %u trace pages on %u cpus\n",
            t_info_pages, pages, num_online_cpus());