diff -r 6ea3db7ae24d tools/libxc/xc_tbuf.c
--- a/tools/libxc/xc_tbuf.c	Wed Jan 30 09:59:27 2008 +0000
+++ b/tools/libxc/xc_tbuf.c	Thu Jan 31 11:31:40 2008 +0800
@@ -103,7 +103,7 @@ int xc_tbuf_set_cpu_mask(int xc_handle, 
     sysctl.interface_version = XEN_SYSCTL_INTERFACE_VERSION;
     sysctl.u.tbuf_op.cmd  = XEN_SYSCTL_TBUFOP_set_cpu_mask;
 
-    bitmap_64_to_byte(bytemap, &mask64, sizeof (mask64));
+    bitmap_64_to_byte(bytemap, &mask64, sizeof (mask64) * 8);
 
     set_xen_guest_handle(sysctl.u.tbuf_op.cpu_mask.bitmap, bytemap);
     sysctl.u.tbuf_op.cpu_mask.nr_cpus = sizeof(bytemap) * 8;