今天编译了一下Xen，发现编出来的xen是32位的可执行程序，而xen-syms却是64位的：
Linux:~/Downloads/xen-4.6.0/xen # file xen
xen: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), statically linked, stripped
Linux:~/Downloads/xen-4.6.0/xen # file xen-syms
xen-syms: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped
Roger在邮件里给了答案：
The Xen entry point is in 32bits (because that’s what the multiboot specification requires). Xen then jumps into long mode (64bits) by itself, so there’s only a very small amount of 32bit code that’s used as a trampoline.
原来是为了支持multiboot，xen实际运行以后还是64位程序。
参考资料：
[Xen-users] Why the built xen file is 32-bit on 64-bit OS?。