follow the steps below:

[root@spark ~]# tar zxf mpich2-1.2.1.tar.gz
[root@spark ~]# cd mpich2-1.2.1
[root@spark mpich2-1.2.1]# patch -p1 < ~/xen-mpich2-1.2.1.patch

if everything is ready, you can configure and make mpich2-1.2.1 as usual.