今天在网上找到一篇文档，描述了SLES12版本上Xen的一些变化：
（1）grub2
SLES12上使用grub2配置Xen参数。以配置dom0_max_vcpus为例：
a）SLES11使用/boot/grub/menu.lst（参考这里）；
b）SLES12使用/etc/default/grub（参考这里）。
（2）xm/xend已经废弃了，要使用xl/libxl。下图做了一个简单对比：
（3）SLES12 Xen Stack：
• Xen 4.4.1
• kernel 3.12.x
• libvirt 1.2.5
• virt-install 1.1.x, vm-install 1.x.x
• virt-manager 1.1.x
这两周一直在crash邮件列表里讨论如何在SuSE Xen上使用crash调试Dom0 kernel。邮件来来回回讨论很多（参见这里），最后还发现了一个bug。细节不说了，把最后的结果总结一下：
（1）由于SuSE kerenl默认编译打开CONFIG_STRICT_DEVMEM编译开关，所以crash工具无法完全访问/dev/mem，可以使用/proc/kcore作为代替；
（2）SuSE带有crash.ko驱动（位于：“/lib/modules/uname -r/updates/crash.ko”），但默认没有安装，可以自己手动安装（使用insmod命令），然后就可以使用了：
# crash
crash 7.1.3
Copyright (C) 2002-2014 Red Hat, Inc.
Copyright (C) 2004, 2005, 2006, 2010 IBM Corporation
Copyright (C) 1999-2006 Hewlett-Packard Co
Copyright (C) 2005, 2006, 2011, 2012 Fujitsu Limited
Copyright (C) 2006, 2007 VA Linux Systems Japan K.K.
Copyright (C) 2005, 2011 NEC Corporation
Copyright (C) 1999, 2002, 2007 Silicon Graphics, Inc.
Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc.
This program is free software, covered by the GNU General Public License,
and you are welcome to change it and/or distribute copies of it under
certain conditions. Enter "help copying" to see the conditions.
This program has absolutely no warranty. Enter "help warranty" for details.
crash: /boot/xen-4.5.gz: original filename unknown
Use "-f /boot/xen-4.5.gz" on command line to prevent this message.
WARNING: machine type mismatch:
crash utility: X86_64
/var/tmp/xen-4.5.gz_ud3IRy: X86
crash: /boot/symtypes-3.12.49-6-default.gz: original filename unknown
Use "-f /boot/symtypes-3.12.49-6-default.gz" on command line to
prevent this message.
crash: /boot/symvers-3.12.49-6-default.gz: original filename unknown
Use "-f /boot/symvers-3.12.49-6-default.gz" on command line to
prevent this message.
GNU gdb (GDB) 7.6
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu"...
KERNEL: /boot/vmlinux-3.12.49-6-xen.gz
DEBUGINFO: /usr/lib/debug/boot/vmlinux-3.12.49-6-xen.debug
DUMPFILE: /dev/crash
CPUS: 128
DATE: Fri Nov 20 06:55:06 2015
UPTIME: 18:51:36
LOAD AVERAGE: 1.76, 1.48, 1.21
TASKS: 1230
NODENAME: dl980-5
RELEASE: 3.12.49-6-xen
VERSION: #1 SMP Mon Oct 26 16:05:37 UTC 2015 (11560c3)
MACHINE: x86_64 (1995 Mhz)
MEMORY: 125.9 GB
PID: 6618
COMMAND: "crash"
TASK: ffff881ea93b2140 [THREAD_INFO: ffff881e869f2000]
CPU: 112
STATE: TASK_RUNNING (ACTIVE)
以下摘自Xen邮件列表：
Hi all,
Since I am a newbie of Xen, this may be a dumb question. On my SuSE Xen, I find the following 4 “Xen” files:
-rw-r–r– 1 root rootÂÂÂ881967 Oct 27 05:46 xen-4.5.110-1.gz
-rw-r–r– 1 root root 16673080 Oct 27 05:46 xen-syms-4.5.110-1
-rw-r–r– 1 root rootÂÂÂ893428 Oct 27 05:45 xen-dbg-4.5.110-1.gz
-rw-r–r– 1 root root 16124144 Oct 27 05:45 xen-syms-dbg-4.5.110-1Could anyone give a detailed explanation of the functions and differences about the 4 files?
TL;DR: Use xen-04.5.1_1-1.gz unless someone asks you to do otherwise or you are tracking down a bug.
xen.gz is the main/regular/normal release build of the Xen binary (e.g. the thing which you can boot). This is the thing you would boot on a regular production Xen system.
xen-syms.gz is the unstripped version of xen.gz, i.e. with all the ELF debug information present. You can e.g. run gdb on it to disassemble things if you are tracking down a bug. This image is not bootable.
The -dbg variants are not something which upstream produces, but I would assume that they are build with the debug=y, which means they will contain extra ASSERT statements and other things which aid debugging possibly at the expense of performance. Release builds (xen.gz et al) are build with debug=n. Like the release builds the xen-*-dbg ones come in the bootable (xen-dbg.gz) and xen-syms-dbg.gz variants, with the same distinction (the former is bootable, the latter is for running gdb on)
概括一下，xen文件是可以bootable的，也是用来启动xen系统的可执行文件。xen-sym是xen的保留所有调试信息的的版本，不可以bootable，仅用于调试功能。xen-*-dbg则是xen和xen-sym对应的加了很多调试代码的版本。
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?。
Xen的虚拟化环境结构如下图所示：
Xen hypervisor：直接运行在硬件上，它负责CPU调度和为虚拟机划分内存。 Xen hypervisor不光为运行之上的虚拟机抽象出硬件，还会控制虚拟机的运行。
Domain 0：Xen hypervisor启动的第一个，拥有特权的虚拟机：比如可以直接访问硬件，启动其它虚拟机等。
Domain U：其它无特权的虚拟机，可以为PV或HM Guest。
以支持Xen的Suse为例，/boot/grub/menu.lst文件如下：
title Xen -- SUSE Linux Enterprise Server 11 SP4 - 3.0.101-63
root (hd0,1)
kernel /boot/xen.gz vga=mode-0x314
module /boot/vmlinuz-3.0.101-63-xen root=/dev/disk/by-id/ata-VBOX_HARDDISK_VBe358a50a-9595120d-part2 resume=/dev/disk/by-id/ata-VBOX_HARDDISK_VBe358a50a-9595120d-part1 splash=silent showopts vga=0x314
module /boot/initrd-3.0.101-63-xen
可以看到，实际运行的kernel是/boot/xen.gz，修改过的kernel（有-xen后缀）和initial ramdisk都是/boot/xen.gz的模块。
参考资料：
How does Xen work？；