xentrace: fix t_info_pages calculation for the default case

(xen-unstable changeset 23091:67632e5cf652)

The default tracebuffer size of 32 pages was not tested with the previous patch.
As a result, t_info_pages will become zero and alloc_xenheap_pages() fails.
Catch this case and allocate at least one page.

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

---
 xen/common/trace.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 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
@@ -33,6 +33,8 @@
 #include <asm/atomic.h>
 #include <public/sysctl.h>
 
+#define PFN_UP(x)     (((x) + PAGE_SIZE-1) >> PAGE_SHIFT)
+
 #ifdef CONFIG_COMPAT
 #include <compat/trace.h>
 #define xen_t_buf t_buf
@@ -109,6 +111,7 @@ static int calculate_tbuf_size(unsigned
 {
     struct t_buf dummy;
     typeof(dummy.prod) size;
+    unsigned int t_info_words, t_info_bytes;
 
     /* force maximum value for an unsigned type */
     size = -1;
@@ -122,11 +125,9 @@ static int calculate_tbuf_size(unsigned
         pages = size;
     }
 
-    t_info_pages = num_online_cpus() * pages + t_info_first_offset;
-    t_info_pages *= sizeof(uint32_t);
-    t_info_pages /= PAGE_SIZE;
-    if ( t_info_pages % PAGE_SIZE )
-        t_info_pages++;
+    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);
     return pages;
 }