diff --git a/Makefile b/Makefile
index fea072d..92b2727 100644
--- a/Makefile
+++ b/Makefile
@@ -7,6 +7,7 @@
 ARCH ?= aarch64
 PLAT ?= qemu
 DEBUG ?= 1
+CROSS_COMPILE ?= aarch64-linux-gnu-
 NAME := hafnium
 
 OUT := $(ROOT_DIR)out/$(ARCH)/$(PLAT)
@@ -33,13 +34,10 @@
 GLOBAL_OFFSET_SRCS :=
 $(foreach mod,$(MODULES),$(eval $(call include_module,$(mod))))
 
-TARGET := aarch64-linux-gnu
-CROSS_PREFIX := $(TARGET)-
-
 ifeq ($(CLANG),1)
-  CC := clang -target $(TARGET)
+  CC := clang -target $(patsubst %-,%,$(CROSS_COMPILE))
 else
-  CC := $(CROSS_PREFIX)gcc
+  CC := $(CROSS_COMPILE)gcc
 endif
 
 #
@@ -136,11 +134,11 @@
 #
 $(OUT)/$(NAME): $(GLOBAL_OBJS) $(ROOT_DIR)src/$(NAME).ld
 	$(info LD $(ROOT_DIR)src/$(NAME).ld)
-	@$(CROSS_PREFIX)ld -g -pie $(GLOBAL_OBJS) -T$(ROOT_DIR)src/$(NAME).ld --defsym PREFERRED_LOAD_ADDRESS=$(LOAD_ADDRESS) -o $@
+	@$(CROSS_COMPILE)ld -g -pie $(GLOBAL_OBJS) -T$(ROOT_DIR)src/$(NAME).ld --defsym PREFERRED_LOAD_ADDRESS=$(LOAD_ADDRESS) -o $@
 
 $(OUT)/$(NAME).bin: $(OUT)/$(NAME)
 	$(info OBJCOPY $@)
-	@$(CROSS_PREFIX)objcopy -O binary $< $@
+	@$(CROSS_COMPILE)objcopy -O binary $< $@
 
 clean:
 	rm -rf $(ROOT_DIR)out
diff --git a/README.md b/README.md
index db297d0..276e657 100644
--- a/README.md
+++ b/README.md
@@ -1,8 +1,120 @@
 # Hafnium
 
+Hafnium is a hypervisor, initially supporting aarch64 (64-bit ARMv8 CPUs).
+
+## Getting the source code
+
 ``` shell
 mkdir hafnium
 cd hafnium
 repo init -u sso://hafnium/manifest
 repo sync -j4
 ```
+
+## Building
+
+### One-time setup
+You will need a cross-compilation toolchain, for example, `aarch64-linux-gnu`, which can be installed with `apt-get` using the following command-line:
+``` shell
+sudo apt-get install gcc-aarch64-linux-gnu
+```
+
+Makefiles use the `CROSS_COMPILE` variable to specify the prefix of the tools; in the case above it is `aarch64-linux-gnu-` (note the trailing dash). In the examples that follow, we will assume that this toolchain is installed and available for use.
+
+### Compiling the hypervisor
+
+The hypervisor proper is in the `hafnium` subdirectory. It can be built for QEMU using the following command:
+
+``` shell
+CROSS_COMPILE=aarch64-linux-gnu- make
+```
+
+The compiled binary is stored in `out/aarch64/qemu/hafnium.bin`.
+
+## Running on QEMU
+
+You will need at least version 2.9 for QEMU. The following command line can be used to run Hafnium on it:
+
+``` shell
+qemu-system-aarch64 -M virt -cpu cortex-a57 -nographic -machine virtualization=true -kernel out/aarch64/qemu/hafnium.bin
+```
+
+Though it is admittedly not very useful because it doesn't have any virtual machines to run. The next sections describe how to build an image that can be run using the following command-line:
+``` shell
+qemu-system-aarch64 -M virt -cpu cortex-a57 -nographic -machine virtualization=true -kernel out/aarch64/qemu/hafnium.bin -initrd initrd.img -append "rdinit=/sbin/init"
+```
+
+## Hafnium RAM disk
+
+Hafnium expects to find the following files in the root directory of its ramdisk:
+
+* `vmlinuz` -- the kernel of the primary VM.
+* `initrd.img` -- the initial ramdisk of the primary VM.
+* `vms.txt` -- optionally describes the secondary VMs.
+* kernels for the secondary VMs, whose names vary and are described in `vms.txt`.
+
+### Format of `vms.txt` file
+The format is currently one line per secondary VM, with the following format:
+``` shell
+<memory-size-in-bytes> <number-of-cpus> <kernel-filename>
+```
+
+For example, the following defines two secondary VMs, the first one with 1MB of memory, 2 CPUs and kernel image called `kernel0`, while the second one has 2MB of memory, 4 CPUs and a kernel image called  `kernel1`.
+``` shell
+1048576 2 kernel0
+2097152 4 kernel1
+```
+
+### Creating a hafnium RAM disk
+
+Assuming that a subdirectory called `initrd` contains the files listed in the previous section, we can build `initrd.img` with the following command:
+``` shell
+cd initrd; find . | cpio -o > ../initrd.img; cd -
+```
+
+## Primary VM
+
+### Kernel
+
+The linux kernel for the primary VM can be built using the following command-line:
+``` shell
+git clone https://github.com/torvalds/linux.git
+cd linux
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make defconfig
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j24
+```
+
+The compiled image is stored in `arch/arm64/boot/Image`. This should be copied to the Hafnium ramdisk's root as `vmlinuz`.
+
+### Busybox RAM disk
+
+An initial ramdisk for the primary VM containing busybox can be built with the following commands:
+
+``` shell
+git clone git://busybox.net/busybox.git
+cd busybox
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make defconfig
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make menuconfig
+```
+
+At this point you should ensure that the option `Build static binary (no shared libs)` under `Settings` is selected. Then you can proceed with the following commands:
+
+``` shell
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j24
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make install
+cd _install
+mkdir proc
+mkdir sys
+mkdir -p etc/init.d
+cat <<EOF > etc/init.d/rcS
+#!bin/sh
+mount -t proc none /proc
+mount -t sysfs none /sys
+EOF
+chmod u+x etc/init.d/rcS
+grep -v tty ../examples/inittab > ./etc/inittab
+find . | cpio -o -H newc | gzip > ../initrd.img
+cd -
+```
+
+The resulting file is `initrd.img`. It should be copied to the Hafnium ramdisk's root.