Virtual TPM interface for Xen
  
  Authors: Matthew Fioravante (JHUAPL), Daniel De Graaf (NSA)
  
  This document describes the virtual Trusted Platform Module (vTPM) subsystem for
  Xen. The reader is assumed to have familiarity with building and installing Xen,
  Linux, and a basic understanding of the TPM and vTPM concepts.
  
  INTRODUCTION
  
  The goal of this work is to provide a TPM functionality to a virtual guest
  operating system (in Xen terms, a DomU).  This allows programs to interact with
  a TPM in a virtual system the same way they interact with a TPM on the physical
  system.  Each guest gets its own unique, emulated, software TPM.  However, each
  of the vTPM's secrets (Keys, NVRAM, etc) are managed by a vTPM Manager domain,
  which seals the secrets to the Physical TPM.  If the process of creating each of
  these domains (manager, vTPM, and guest) is trusted, the vTPM subsystem extends
  the chain of trust rooted in the hardware TPM to virtual machines in Xen. Each
  major component of vTPM is implemented as a separate domain, providing secure
  separation guaranteed by the hypervisor. The vTPM domains are implemented in
  mini-os to reduce memory and processor overhead.
  
  This mini-os vTPM subsystem was built on top of the previous vTPM work done by
  IBM and Intel corporation.
  
  
  DESIGN OVERVIEW
  ---------------
  
  The architecture of vTPM is described below:
  
  +------------------+
  |    Linux DomU    | ...
  |       |  ^       |
  |       v  |       |
  |   xen-tpmfront   |
  +------------------+
          |  ^
          v  |
  +------------------+
  | mini-os/tpmback  |
  |       |  ^       |
  |       v  |       |
  |  vtpm-stubdom    | ...
  |       |  ^       |
  |       v  |       |
  | mini-os/tpmfront |
  +------------------+
          |  ^
          v  |
  +------------------+
  | mini-os/tpmback  |
  |       |  ^       |
  |       v  |       |
  | vtpmmgr-stubdom  |
  |       |  ^       |
  |       v  |       |
  | mini-os/tpm_tis  |
  +------------------+
          |  ^
          v  |
  +------------------+
  |   Hardware TPM   |
  +------------------+
  
   * Linux DomU: The Linux based guest that wants to use a vTPM. There may be
  	       more than one of these.
  
   * xen-tpmfront.ko: Linux kernel virtual TPM frontend driver. This driver
                      provides vTPM access to a Linux-based DomU.
  
   * mini-os/tpmback: Mini-os TPM backend driver. The Linux frontend driver
  		    connects to this backend driver to facilitate communications
  		    between the Linux DomU and its vTPM. This driver is also
  		    used by vtpmmgr-stubdom to communicate with vtpm-stubdom.
  
   * vtpm-stubdom: A mini-os stub domain that implements a vTPM. There is a
  		 one to one mapping between running vtpm-stubdom instances and
                   logical vtpms on the system. The vTPM Platform Configuration
                   Registers (PCRs) are normally all initialized to zero.
  
   * mini-os/tpmfront: Mini-os TPM frontend driver. The vTPM mini-os domain
  		     vtpm-stubdom uses this driver to communicate with
  		     vtpmmgr-stubdom. This driver is also used in mini-os
  		     domains such as pv-grub that talk to the vTPM domain.
  
   * vtpmmgr-stubdom: A mini-os domain that implements the vTPM manager. There is
  		    only one vTPM manager and it should be running during the
  		    entire lifetime of the machine.  This domain regulates
  		    access to the physical TPM on the system and secures the
  		    persistent state of each vTPM.
  
   * mini-os/tpm_tis: Mini-os TPM version 1.2 TPM Interface Specification (TIS)
                      driver. This driver used by vtpmmgr-stubdom to talk directly to
                      the hardware TPM. Communication is facilitated by mapping
                      hardware memory pages into vtpmmgr-stubdom.
  
   * Hardware TPM: The physical TPM that is soldered onto the motherboard.
  
  
  INTEGRATION WITH XEN
  --------------------
  
  Support for the vTPM driver was added in Xen using the libxl toolstack in Xen
  4.3.  See the Xen documentation (docs/misc/vtpm.txt) for details on setting up
  the vTPM and vTPM Manager stub domains.  Once the stub domains are running, a
  vTPM device is set up in the same manner as a disk or network device in the
  domain's configuration file.
  
  In order to use features such as IMA that require a TPM to be loaded prior to
  the initrd, the xen-tpmfront driver must be compiled in to the kernel.  If not
  using such features, the driver can be compiled as a module and will be loaded
  as usual.