docs: add AArch64 build docs

docs/aarch64_build.md

@@ -0,0 +1,90 @@
+# Building IA2 and compartmentalized programs for AArch64
+
+Because the AArch64 port of IA2 requires changes to compiler code generation
+(reserving the x18 register and instrumenting memory accesses),
+all code executing in a compartmentalized process must be built with a modified toolchain.
+
+To do so, the first step is to build the modified toolchain itself.
+The toolchain is available at <https://github.com/immunant/llvm-project/tree/fw/instrument-llvm-ir>,
+and can be built by running the [`build.sh`](https://github.com/immunant/llvm-project/blob/fw/instrument-llvm-ir/build.sh) script provided.
+This will build LLVM, the Clang compiler, and then use the built Clang compiler to build the LLVM Builtins library.
+
+## Using the modified toolchain
+
+Because the toolchain instruments all memory accesses by applying the tag stored in the x18 register,
+the `-ffixed-x18` flag must always be passed when compiling code with it.
+
+This also serves to ensure that the modified toolchain is not being used mistakenly, e.g. for host code.
+
+## Building runtime libraries
+
+Beneath the C standard library itself, the compiler toolchain implements various functionality
+(such as process setup and pre-termination callbacks) using "runtime libraries".
+These libraries are built once and distributed with the compiler to link into
+each program that the compiler builds.
+To avoid code not compiled by the modified toolchain being linked into compartmentalized binaries,
+a separate copy of these runtime libraries must be built by the modified toolchain.
+This is achieved using the [`cross-build-rtlibs.sh`](https://github.com/immunant/llvm-project/blob/fw/instrument-llvm-ir/cross-build-rtlibs.sh) script in the modified LLVM branch.
+
+## Building IA2 using the toolchain and libraries
+
+Once the modified toolchain and runtime libraries have been built, they must be used to build IA2.
+
+For later convenience, we define `toolchain_dir` to point to our compiler toolchain and custom build of runtime libraries.
+Specifically, `toolchain_dir` should be a directory containing our Clang binary at the relative path `bin/clang-19`.
+For example, if we cloned `llvm-project` adjacent to IA2:
+```
+toolchain_dir=$(pwd)/../llvm-project/build
+```
+
+If the host is an x86_64 system, this consitutes a cross-compiling build,
+so appropriate flags must be passed to specify the target architecture variant and toolchain paths:
+```
+cross_flags="-isystem /usr/aarch64-linux-gnu/include --gcc-toolchain=/usr -march=armv8.5-a+memtag"
+```
+
+We must pass the `-ffixed-x18` argument when using the modified toolchain:
+```
+cross_flags+=" -ffixed-x18"
+```
+
+And the compiler must be directed to use the LLVM implementation of the C runtime libraries and C++ standard library,
+which must be accompanied by its headers:
+```
+cross_flags+=" --rtlib=compiler-rt --stdlib=libc++ -I${rtlibs_dir}/include/c++/v1"
+```
+
+We also define an `rtlibs_dir` variable to point to the path at which we compiled the C runtime libraries.
+This directory should contain `compiler-rt/lib/linux/crtbeginS.o`.
+Given the runtime libraries built at the path `${rtlibs_dir}`,
+the link stage of compilation must be directed to search for the runtime libraries themselves
+with the `-B` flag (for startup files) and `-L` flag (for the C++ standard library):
+
+```
+cross_link_flags="-B${rtlibs_dir}/compiler-rt/lib/linux -L${rtlibs_dir}/lib"
+```
+
+In addition, the Clang toolchain must be directed to link its own runtime libraries rather than system-provided ones:
+```
+cross_link_flags+=" --rtlib=compiler-rt --unwindlib=libunwind"
+```
+
+The linker must be directed to sysroot for other libraries to be used in the build:
+```
+export LDFLAGS="-L/usr/aarch64-linux-gnu/lib"
+```
+
+Finally, we can use these flags for compilation with CMake, adding these flags to our `cmake` invocation:
+
+```
+cmake .. \
+-DCMAKE_CROSSCOMPILING_EMULATOR=${{ github.workspace }}/qemu/build/qemu-aarch64 \
+-DCMAKE_TOOLCHAIN_FILE=../cmake/aarch64-toolchain.cmake \
+-DCMAKE_C_COMPILER=${toolchain_dir}/bin/clang-19 \
+-DCMAKE_CXX_COMPILER=${toolchain_dir}/bin/clang-19 \
+-DCMAKE_C_FLAGS="$cross_flags" \
+-DCMAKE_CXX_FLAGS="$cross_flags" \
+-DCMAKE_EXE_LINKER_FLAGS="$cross_link_flags -lm -Wl,-rpath-link,${rtlibs_dir}/lib" \
+-DCMAKE_SHARED_LINKER_FLAGS="$cross_link_flags -Wl,-rpath,${rtlibs_dir}/lib" \
+-G Ninja
+```