[PATCH v2] docs/devel: Add cross-compiling doc

[Andrew Jones <ajones@ventanamicro.com>]

Add instructions for how to cross-compile QEMU for RISC-V. The
file is named generically because there's no reason not to collect
other architectures steps into the same file, especially because
several subsections like those for cross-compiling QEMU dependencies
using meson and a cross-file could be shared. Additionally, other
approaches to creating sysroots, such as with debootstrap, may be
documented in this file in the future.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
---
 docs/devel/cross-compiling.rst | 221 +++++++++++++++++++++++++++++++++
 docs/devel/index-build.rst     |   1 +
 2 files changed, 222 insertions(+)
 create mode 100644 docs/devel/cross-compiling.rst

diff --git a/docs/devel/cross-compiling.rst b/docs/devel/cross-compiling.rst
new file mode 100644
index 000000000000..1b988ba54e4c
--- /dev/null
+++ b/docs/devel/cross-compiling.rst
@@ -0,0 +1,221 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+====================
+Cross-compiling QEMU
+====================
+
+Cross-compiling QEMU first requires the preparation of a cross-toolchain
+and the cross-compiling of QEMU's dependencies. While the steps will be
+similar across architectures, each architecture will have its own specific
+recommendations. This document collects architecture-specific procedures
+and hints that may be used to cross-compile QEMU, where typically the host
+environment is x86.
+
+RISC-V
+======
+
+Toolchain
+---------
+
+Select a root directory for the cross environment
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Export an environment variable pointing to a root directory
+for the cross environment. For example, ::
+
+  $ export PREFIX="$HOME/opt/riscv"
+
+Create a work directory
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Tools and several components will need to be downloaded and built. Create
+a directory for all the work, ::
+
+  $ export WORK_DIR="$HOME/work/xqemu"
+  $ mkdir -p "$WORK_DIR"
+
+Select and prepare the toolchain
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Select a toolchain such as [riscv-toolchain]_ and follow its instructions
+for building and installing it to ``$PREFIX``, e.g. ::
+
+  $ cd "$WORK_DIR"
+  $ git clone https://github.com/riscv/riscv-gnu-toolchain
+  $ cd riscv-gnu-toolchain
+  $ ./configure --prefix="$PREFIX"
+  $ make -j$(nproc) linux
+
+Set the ``$CROSS_COMPILE`` environment variable to the prefix of the cross
+tools and add the tools to ``$PATH``, ::
+
+$ export CROSS_COMPILE=riscv64-unknown-linux-gnu-
+$ export PATH="$PREFIX/bin:$PATH"
+
+Also set ``$SYSROOT``, where all QEMU cross-compiled dependencies will be
+installed. The toolchain installation likely created a 'sysroot' directory
+at ``$PREFIX/sysroot``, which is the default location for most cross
+tools, making it a good location, ::
+
+  $ mkdir -p "$PREFIX/sysroot"
+  $ export SYSROOT="$PREFIX/sysroot"
+
+Create a pkg-config wrapper
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The build processes of QEMU and some of its dependencies depend on
+pkg-config. Create a wrapper script for it which works for the cross
+environment: ::
+
+  $ cat <<EOF >"$PREFIX/bin/${CROSS_COMPILE}pkg-config"
+  #!/bin/sh
+
+  [ "\$SYSROOT" ] || exit 1
+
+  export PKG_CONFIG_PATH=
+  export PKG_CONFIG_LIBDIR="\${SYSROOT}/usr/lib/pkgconfig:\${SYSROOT}/usr/lib64/pkgconfig:\${SYSROOT}/usr/share/pkgconfig"
+
+  exec pkg-config "\$@"
+  EOF
+  $ chmod +x "$PREFIX/bin/${CROSS_COMPILE}pkg-config"
+
+Create a cross-file for meson builds
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+meson setup, used by some of QEMU's dependencies, needs a "cross-file" to
+configure the cross environment. Create one, ::
+
+  $ cd "$WORK_DIR"
+  $ cat <<EOF >cross_file.txt
+  [host_machine]
+  system = 'linux'
+  cpu_family = 'riscv64'
+  cpu = 'riscv64'
+  endian = 'little'
+
+  [binaries]
+  c = '${CROSS_COMPILE}gcc'
+  cpp = '${CROSS_COMPILE}g++'
+  ar = '${CROSS_COMPILE}ar'
+  ld = '${CROSS_COMPILE}ld'
+  objcopy = '${CROSS_COMPILE}objcopy'
+  strip = '${CROSS_COMPILE}strip'
+  pkgconfig = '${CROSS_COMPILE}pkg-config'
+  EOF
+
+Cross-compile dependencies
+--------------------------
+
+glibc
+^^^^^
+
+If [riscv-toolchain]_ was selected for the toolchain then this step is
+already complete and glibc has already been installed into ``$SYSROOT``.
+Otherwise, cross-compile glibc and install it to ``$SYSROOT``.
+
+libffi
+^^^^^^
+
+::
+
+  $ cd "$WORK_DIR"
+  $ git clone https://gitlab.freedesktop.org/gstreamer/meson-ports/libffi.git
+  $ cd libffi
+  $ meson setup --cross-file ../cross_file.txt --prefix="$SYSROOT/usr" _build
+  $ ninja -C _build
+  $ ninja -C _build install
+
+*Building libffi seperately avoids a compilation error generated when
+building it as a subproject of glib.*
+
+glib
+^^^^
+
+::
+
+  $ cd "$WORK_DIR"
+  $ git clone https://github.com/GNOME/glib.git
+  $ cd glib
+  $ meson setup --cross-file ../cross_file.txt --prefix="$SYSROOT/usr" _build
+  $ ninja -C _build
+  $ ninja -C _build install
+
+libslirp [optional]
+^^^^^^^^^^^^^^^^^^^
+
+::
+
+  $ cd "$WORK_DIR"
+  $ git clone https://gitlab.com/qemu-project/libslirp.git
+  $ cd libslirp
+  $ meson setup --cross-file ../cross_file.txt --prefix="$SYSROOT/usr" _build
+  $ ninja -C _build
+  $ ninja -C _build install
+
+pixman
+^^^^^^
+
+First ensure the 'libtool' package is installed, e.g.
+``sudo dnf install libtool`` or ``sudo apt install libtool``
+
+::
+
+  $ cd "$WORK_DIR"
+  $ git clone https://gitlab.freedesktop.org/pixman/pixman
+  $ cd pixman
+  $ ./autogen.sh
+  $ ./configure --prefix="$SYSROOT/usr" --host=riscv64-unknown-linux-gnu
+  $ make -j$(nproc)
+  $ make install
+
+Cross-compile QEMU
+------------------
+
+::
+
+  $ cd "$WORK_DIR"
+  $ git clone https://gitlab.com/qemu-project/qemu.git
+  $ cd qemu
+  $ mkdir -p build/install_dir
+  $ cd build
+  $ ../configure --target-list=riscv64-softmmu --cross-prefix=$CROSS_COMPILE --prefix="$PWD/install_dir"
+  $ make -j$(nproc)
+  $ make install
+
+*Cross-compiling QEMU with different configurations may require more
+dependencies to be built and installed in the sysroot.*
+
+Running QEMU
+------------
+
+``build/install_dir`` may now be copied to the target and its bin
+directory may be added to the target user's PATH. Prior to running
+QEMU, ensure all the libraries it depends on are present, ::
+
+  $ ldd /path/to/bin/qemu-system-riscv64
+
+For example, it may necessary to install zlib libraries, e.g.
+``sudo dnf install zlib-devel`` or ``sudo apt install zlib1g-dev``
+
+Subsequent QEMU Cross-compiling
+-------------------------------
+
+Unless it's necessary to update and recompile the toolchain or
+dependencies, then most steps do not need to be repeated for subsequent
+compiles. Simply ensure the toolchain is in ``$PATH``, ``$SYSROOT`` points
+at the sysroot, and then follow the QEMU cross-compile steps in
+"Cross-compile QEMU". For example, ::
+
+  $ export PATH="$HOME/opt/riscv/bin:$PATH"
+  $ export SYSROOT="$HOME/opt/riscv/sysroot"
+  $ cd /path/to/qemu
+  $ mkdir -p build/install_dir
+  $ cd build
+  $ ../configure --target-list=riscv64-softmmu --cross-prefix=riscv64-unknown-linux-gnu- --prefix="$PWD/install_dir"
+  $ make -j
+  $ make install
+
+References
+----------
+
+.. [riscv-toolchain] https://github.com/riscv/riscv-gnu-toolchain
diff --git a/docs/devel/index-build.rst b/docs/devel/index-build.rst
index 57e8d39d9856..d3c85927be24 100644
--- a/docs/devel/index-build.rst
+++ b/docs/devel/index-build.rst
@@ -9,6 +9,7 @@ the basics if you are adding new files and targets to the build.
    :maxdepth: 3
 
    build-system
+   cross-compiling
    kconfig
    testing
    acpi-bits
-- 
2.41.0