tutorial: add QEMU details

Recently, I had to dig into how to set up QEMU with a virtualized TPM2.0
device to debug a potential issue in the in-kernel resource manager
(/dev/tpmrm0). Their is a quite a bit of information on setting up QEMU,
and a TPM device, it seemed to be lacking in setting up the TPM2.0
device. The crux was the subtle and easy to overlook --tpm2 option
to the SWTPM.

Build a simple tutorial to highlight a setup using TPM2.0.

Signed-off-by: William Roberts <[email protected]>

_posts/2020-10-19-QEMU.md

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+# Introduction
+End-to-end development with physical hardware can be challenging due to a myriad
+of factors. Things like persistent state, physical wear, slow and difficult
+to update hardware bugs, lagging features, etc can pose additional hurdles to
+development tasks.
+
+A potential way to overcome this, is to use [QEMU](https://www.qemu.org) instance with an attached
+virtualized TPM2.0 device. This device is made available to the guest OS, and
+with the appropriate versions of Linux, will expose the familiar /dev/tpm0
+and /dev/tpmrm0 interfaces.
+
+In this brief tutorial, we provide instructions on how to build such a system by leveraging other documentation as required.
+
+# Prerequisites
+Prior art does exist on this topic, and details used in this tutorial have references from
+the following resources:
+  - https://www.qemu.org/docs/master/specs/tpm.html
+  - https://graspingtech.com/ubuntu-desktop-18.04-virtual-machine-macos-qemu/
+
+## Install the proper TPM2.0 Simulator
+
+In this tutorial, we demonstrate how to leverage the [swtpm](https://github.com/stefanberger/swtpm) as the TPM simulator.
+The project wiki has instructions for building and installing the simulator and its dependency, [libtpms](https://github.com/stefanberger/libtpms).
+
+## Install QEMU
+
+Next, you need to install QEMU. This is operating system dependent. Details on installing
+QEMU can be found by visiting their website:
+  - https://www.qemu.org/
+
+**The minimum version of QEMU to support this is 4.0. In this tutorial, the author tested with version 5.2.**
+
+# Install The Guest OS
+
+The author installed Ubuntu 20.04, so the commands will be specific to that ISO, but another ISO
+could be substituted. Additionally, the naming convention on things like hard-drive could be changed
+to reflect your environment more closely.
+
+Install the guest OS. This will be guest-OS specific. The general commands are to build
+a virtual disk:
+```console
+qemu-img create -f qcow2 ubuntu-20.04-amd64.img 30G
+```
+
+Then attach it to a VM and start it with the installation media, usually an ISO:
+```console
+qemu-system-x86_64 -hda ~/qemu-images/ubuntu-20.04-amd64.img -boot d -cdrom ~/Downloads/ubuntu-20.04.1-desktop-amd64.iso -m 2048 -enable-kvm
+```
+
+# Start the Guest with a TPM2.0 Device
+
+Now start the guest with a virtualized TPM2.0 device. To do this, one needs to start the SWTPM simulator in tpm2 mode using the option
+`--tpm2`, like so:
+```console
+mkdir /tmp/emulated_tpm
+swtpm socket --tpmstate dir=/tmp/emulated_tpm --ctrl type=unixio,path=/tmp/emulated_tpm/swtpm-sock --log level=20 --tpm2
+```
+
+Then start the guest:
+```console
+qemu-system-x86_64 -hda ~/qemu-images/ubuntu-20.04-amd64.img -boot d -m 2048 -enable-kvm \
+  -chardev socket,id=chrtpm,path=/tmp/emulated_tpm/swtpm-sock \
+  -tpmdev emulator,id=tpm0,chardev=chrtpm -device tpm-tis,tpmdev=tpm0
+```
+
+Now verify that the device nodes are present in the guest VM by opening a console and running the following command:
+```console
+ls /dev/tpm*
+```
+
+You should see `/dev/tpm0` and `/dev/tpmrm0` devices in the output of the `ls` command.
+
+# Conclusion
+
+One of the major benefits to the emulated environment is being able to test end-to-end development without the need for physical
+hardware and it's associated drawbacks across a wide variety of environments. QEMU has the ability to emulate multiple physical
+CPU architectures. Couple that with the ability to install a wide array of operating systems, and you have a flexible system for
+debugging and building new features from the lowest portions of the stack all they way to end client applications.
+
+# Author
+William Roberts