Add qemu.md

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>

devices/qemu.md

+# OP-TEE on QEMU
+
+# Contents
+1. [Introduction](#1-introduction)
+2. [Regular build](#2-regular-build)
+3. [Install files on the device](#3-qemu-console)
+4. [Host-Guest folder sharing in QEMU/QEMUv8 configurations](#4-host-guest-folder-sharing-in-qemuqemuv8-configurations)
+
+# 1. Introduction
+The instructions here will tell how to run OP-TEE using QEMU. We have two
+working configurations one for ARMv7-A and one for ARMv8-A. The major difference
+between the two is the boot procedure. In the ARMv7-A case it's using a bios,
+but on ARMv8-A it uses ARM-TF etc instead.
+
+# 2. Regular build
+As long as you pick either the v7 (default.xml) or the v8 (qemu_v8.xml) the "Get
+and build the solution" in the [README.md] file tells all you need to know to
+build and boot up QEMU.
+
+# 3. QEMU console
+After running `make run` you will end up in the QEMU console and it will also
+spawn two xterm windows. One console containing the UART for secure world and
+one console containing the UART for normal world.
+
+## 3.1 ARMv7-A
+It will stop on the QEMU console, to continue, simply
+```
+(qemu) c
+```
+
+## 3.2 ARMv8-A
+It won't stop, it will just boot up QEMU and OP-TEE.
+
+#4. Host-Guest folder sharing in QEMU/QEMUv8 configurations
+
+To avoid changing rootfs CPIO archive each time you need to add additional files
+to it, you can also use VirtFS QEMU feature to share a folder between the guest
+and host operating systems. To use this feature enable VirtFS QEMU build in
+build/common.mk (set QEMU_VIRTFS_ENABLE ?= y), adjust QEMU_VIRTFS_HOST_DIR and
+rebuild QEMU.
+
+To mount host folder in QEMU, simply run:
+
+```bash
+$ mount_shared <mount_point>
+```
+
+[bios]: https://github.com/linaro-swg/bios_qemu_tz_arm
+[README.md]: https://github.com/OP-TEE/build/README.md