Stage 1：寻找消失的指令集
对比加载新内核前后树莓派3B上执行lscpu的输出：
Before:
Architecture:armv7l
...
Flags:half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm crc32
After:
Architecture:aarch64
...
Flags:fp asimd evtstrm crc32
显然，有许多指令集莫名消失。
笔者使用的aarch-linux-gnu-工具链：
$ aarch64-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=aarch64-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/aarch64-linux-gnu/6/lto-wrapper
Target: aarch64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 6.3.0-18' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-6 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libquadmath --enable-plugin --enable-default-pie --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-arm64-cross/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-arm64-cross --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-arm64-cross --with-arch-directory=aarch64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libgcj --enable-multiarch --enable-fix-cortex-a53-843419 --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=aarch64-linux-gnu --program-prefix=aarch64-linux-gnu- --includedir=/usr/aarch64-linux-gnu/include
Thread model: posix
gcc version 6.3.0 20170516 (Debian 6.3.0-18)
笔者遂使用crosstool-ng制作编译工具链，并惊喜地发现：
$ ls /opt/crosstool-ng/samples/
aarch64-rpi3-linux-gnueabi
...
crosstool-ng-1.23.0-75-ga9f8a8e6中已添加针对树莓派3的支持。
于是笔者开始制作工具链。
选中了linaro分支的gcc。
并按照与前文相似的方法编译内核，然而没有效果，指令集仍未被识别。
Stage 2：使用debootstrap制作64位组件
尽管内核工作于64位模式，但系统大多数组件仍旧处于32位模式下。
我们的目标是引入Debian 9 arm64，自然会想到使用debootstrap。
在树莓派上，给一张SD卡分两个区：
一个在前面，大小100M左右，使用mkfs.vfat
一个占用后面所有的空间，视/boot/cmdline.txt中rootfstype的设置选择分区格式
然后开干：
sudo debootstrap --verbose --arch=arm64 stretch . https://mirrors.ustc.edu.cn/debian/
编辑sources.list：
deb http://mirrors.ustc.edu.cn/debian/ stretch main
deb http://mirrors.ustc.edu.cn/archive.raspberrypi.org/ jessie main non-free contrib
apt.conf：
APT::Default-Release "stretch";
Chroot和之前的准备工作：
sudo mount -t proc proc proc
sudo mount -t sysfs sysfs sys
sudo mount -o bind /dev dev
sudo chroot .
视具体情况安装软件包。
apt install hostapd wpasupplicant udhcpd screen dnsmasq sudo openssh-server firmware-brcm80211 ...
firmware-brcm80211：使系统识别树莓派3板载网卡
hostapd,udhcpd,dnsmasq：启动AP必备
wpasupplicant：用于连接Wifi热点
openssh-server：SSH服务器
sudo,screen：这两个想必大家都听说过吧
退出Chroot环境，复制配置文件及其它核心组件，例如：
/boot
/lib/modules
/etc/passwd
/etc/shadow
/etc/sudoers
/etc/hostapd
/etc/default
/etc/udhcpd.conf
/etc/udev/
/etc/dnsmasq.conf
etc...
注：笔者实际上把整个（除了/etc）64位根文件系统覆盖到了树莓派启动的SD卡上。
成功启动。
但是有烦人的提示：
ERROR: ld.so: object '/usr/lib/arm-linux-gnueabihf/libarmmem.so' from /etc/ld.so.preload cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
解决方案显而易见：
sudo mv /etc/ld.so.preload /etc/ld.so.preload.bak
sudo touch /etc/ld.so.preload
Stage 3：站稳脚跟
执行apt-get update，提示：
Ign:1 https://mirrors.ustc.edu.cn/debian stretch InRelease
Get:2 https://mirrors.ustc.edu.cn/archive.raspberrypi.org jessie InRelease [22.9 kB]
Hit:2 https://mirrors.ustc.edu.cn/archive.raspberrypi.org jessie InRelease
Get:3 https://mirrors.ustc.edu.cn/debian stretch Release [113 kB]
Hit:3 https://mirrors.ustc.edu.cn/debian stretch Release
Can't locate Storable.pm in @INC (you may need to install the Storable module) (@INC contains: /etc/perl /usr/local/lib/aarch64-linux-gnu/perl/5.24.1 /usr/local/share/perl/5.24.1 /usr/lib/aarch64-linux-gnu/perl5/5.24 /usr/share/perl5 /usr/lib/aarch64-linux-gnu/perl/5.24 /usr/share/perl/5.24 /usr/local/lib/site_perl /usr/lib/aarch64-linux-gnu/perl-base) at /usr/bin/apt-show-versions line 37.
BEGIN failed--compilation aborted at /usr/bin/apt-show-versions line 37.
Reading package lists... Done
N: Skipping acquire of configured file 'main/binary-arm64/Packages' as repository 'https://mirrors.ustc.edu.cn/archive.raspberrypi.org jessie InRelease' doesn't support architecture 'arm64'
N: Skipping acquire of configured file 'non-free/binary-arm64/Packages' as repository 'https://mirrors.ustc.edu.cn/archive.raspberrypi.org jessie InRelease' doesn't support architecture 'arm64'
N: Skipping acquire of configured file 'contrib/binary-arm64/Packages' as repository 'https://mirrors.ustc.edu.cn/archive.raspberrypi.org jessie InRelease' doesn't support architecture 'arm64'
N: Skipping acquire of configured file 'ui/binary-arm64/Packages' as repository 'https://mirrors.ustc.edu.cn/archive.raspberrypi.org jessie InRelease' doesn't support architecture 'arm64'
E: Problem executing scripts APT::Update::Post-Invoke-Success 'test -x /usr/bin/apt-show-versions || exit 0 ; apt-show-versions -i'
E: Sub-process returned an error code
笔者尝试执行sudo apt-get install apt-show-versions，问题解决。
小结
路曼曼其修远兮，吾将上下而求索。
目前整个系统仅仅只是能用，还有许多不完美的地方。