xen bootloader
XendBootLoader.py:[*]判读bootloader是否存在
[*] Linux：/usr/bin/pygrub
[*] Windows: /usr/lib/xen/boot/hvmloade
[*]判断 启动设备是否存在，不存在返回：Disk isn't accessible
[*]打开一个管道，用于两个pty进程间通信：/var/run/xend/boot/xenbl.%s
[*]构造命令并在pty子进程中执行：
[*]/usr/bin/pygrub [ --kernel=%s --ramdisk=%s --args=%s--output=%s -q ] path-to-disk
[*]如果子进程（执行bootloader）执行失败，或没有任何输出，则raise VmError, msg，否则返回sxp格式的结果。
源码中的注释：
# We need to present the bootloader's tty as a pty slave that xenconsole
# can access.Since the bootloader itself needs a pty slave,
# we end up with a connection like this:
#
# xenconsole -- (slave pty1 master)(master pty2 slave) -- bootloader
#
# where we copy characters between the two master fds, as well as
# listening on the bootloader's fifo for the results.
# filedescriptors:
# r - input from the bootloader (bootstring output)
# m1 - input/output from/to xenconsole
# m2 - input/output from/to pty that controls the bootloader
/usr/bin/pygrub
pygrub解析传入的分区的结构，如果是一个disk，那么就从disk找到active的分区，如果是分区，那就直接解析分区，然后引导启动。
Usage: /usr/bin/pygrub [-q|--quiet] [-i|--interactive] [--output=] [--kernel=] [--ramdisk=] [--args=] [--entry=]
[*] grub文件路径为：/boot/grub/menu.lst", "/boot/grub/grub.conf" 或 "/grub/menu.lst", "/grub/grub.conf"
[*]使用pygrub，启动分区要放在第一个位置。
手动执行的结果是：
linux (kernel /var/run/xend/boot/boot_kernel.L_jphs)(ramdisk /var/run/xend/boot/boot_ramdisk.9Ed02z)(args "ro root=/dev/sda2 ")
PV-GRUB被认为是pygrub的取代：
PV-GRUB replaces PyGrub to boot domU images safely: it runs the regular grub inside the created domain itself and uses regular domU facilities to read the disk / fetch files from network etc.; it eventually loads the PV kernel and chain-boots it.
kernel = "/usr/lib/xen/boot/pv-grub-x86_32.gz"
extra = "(hd0,0)/grub/menu.lst"
/usr/lib/xen/boot/hvmloader
源码在hvmloader.c中，有待继续研究。
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