update info about build gcc-musl in ubuntu

README.md

@@ -16,8 +16,12 @@ Now it has 3 parts:
 To build biscuit programs, install musl toolchain first:
 
 ```bash
+# for ARCH linux
 $ sudo pacman -Syu musl # archlinux
-$ brew install FileSottile/musl-cross/musl-cross # macOS
+# for macOS
+$ brew install FileSottile/musl-cross/musl-cross 
+# for ubuntu 16.04, we should build gcc-musl for newest musl-1.1.21
+# please see build-gcc-musl.md
 ```
 
 ```bash

build-gcc-musl.md

@@ -0,0 +1,31 @@
+## step 1: build musl-gcc with gcc-6.4+musl-1.1.21
+```
+git clone https://github.com/richfelker/musl-cross-make.git
+cp config.mak.dist config.mak
+
+# add two line in config.mak
+TARGET = x86_64-linux-musl
+OUTPUT = /usr/local
+
+make && sudo make install
+```
+
+## step 2: remove ubuntu musl* packages
+```
+sudo apt remove musl*
+```
+
+## step 3: use x86_64-linux-musl-gcc to rebuild sfsimg
+```
+# change rcore_plus/user/biscuit/CMakeLists.txt LINE:35
+        set(PREFIX          x86_64-linux-musl-)
+
+cd usr && make clean && make sfsimg arch=x86_64
+cd ../kernel && make run arch=x86_64
+```
+
+## final test
+```
+# in qemu
+biscuit/ls ##should work now
+```