[PATCH v5 8/8] docs/nitro-enclave: Documentation for nitro-enclave machine type

[Dorjoy Chowdhury <dorjoychy111@gmail.com>]

Signed-off-by: Dorjoy Chowdhury <dorjoychy111@gmail.com>
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+'nitro-enclave' virtual machine (``nitro-enclave``)
+===================================================
+
+``nitro-enclave`` is a machine type which emulates an ``AWS nitro enclave``
+virtual machine. `AWS nitro enclaves`_ is an `Amazon EC2`_ feature that allows
+creating isolated execution environments, called enclaves, from Amazon EC2
+instances which are used for processing highly sensitive data. Enclaves have
+no persistent storage and no external networking. The enclave VMs are based
+on Firecracker microvm with a vhost-vsock device for communication with the
+parent EC2 instance that spawned it and a Nitro Secure Module (NSM) device
+for cryptographic attestation. The parent instance VM always has CID 3 while
+the enclave VM gets a dynamic CID. Enclaves use an EIF (`Enclave Image Format`_)
+file which contains the necessary kernel, cmdline and ramdisk(s) to boot.
+
+In QEMU, ``nitro-enclave`` is a machine type based on ``microvm`` similar to how
+``AWS nitro enclaves`` are based on ``Firecracker`` microvm. This is useful for
+local testing of EIF files using QEMU instead of running real AWS Nitro Enclaves
+which can be difficult for debugging due to its roots in security. The vsock
+device emulation is done using vhost-user-vsock which means another process that
+can do the userspace emulation, like `vhost-device-vsock`_ from rust-vmm crate,
+must be run alongside nitro-enclave for the vsock communication to work.
+
+``libcbor`` and ``gnutls`` are required dependencies for nitro-enclave machine
+support to be added when building QEMU from source.
+
+.. _AWS nitro enlaves: https://docs.aws.amazon.com/enclaves/latest/user/nitro-enclave.html
+.. _Amazon EC2: https://aws.amazon.com/ec2/
+.. _Enclave Image Format: https://github.com/aws/aws-nitro-enclaves-image-format
+.. _vhost-device-vsock: https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-vsock
+
+Using the nitro-enclave machine type
+------------------------------
+
+Machine-specific options
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+It supports the following machine-specific options:
+
+- nitro-enclave.vsock=string (required) (Id of the chardev from '-chardev' option that vhost-user-vsock device will use)
+- nitro-enclave.id=string (optional) (Set enclave identifier)
+- nitro-enclave.parent-role=string (optional) (Set parent instance IAM role ARN)
+- nitro-enclave.parent-id=string (optional) (Set parent instance identifier)
+
+
+Running a nitro-enclave VM
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+First, run vhost-device-vsock (or a similar tool that supports vhost-user-vsock)
+
+  $ vhost-device-vsock \
+     --vm guest-cid=4,uds-path=/tmp/vm4.vsock,socket=/tmp/vhost4.socket \
+     --vm guest-cid=3,uds-path=/tmp/vm3.vsock,socket=/tmp/vhost3.socket
+
+Then, run the parent VM that has the necessary vsock communication support.
+
+  $ qemu-system-x86_64 -machine q35,memory-backend=mem0 -enable-kvm -m 8G \
+     -nic user,model=virtio -drive file=test_vm.qcow2,media=disk,if=virtio \
+     --display sdl -object memory-backend-memfd,id=mem0,size=8G \
+     -chardev socket,id=char0,reconnect=0,path=/tmp/vhost3.socket \
+     -device vhost-user-vsock-pci,chardev=char0
+
+Inside this VM the necessary applications should be run so that the nitro-enclave
+VM applications' vsock communication works. For example, the nitro-enclave VM's
+init process connects to CID 3 and sends a single byte hello heartbeat (0xB7) to
+let the parent VM know that it booted expecting a heartbeat (0xB7) response.
+
+Now run the nitro-enclave VM using the following command where ``hello.eif`` is
+an EIF file you would use to spawn a real AWS nitro enclave virtual machine:
+
+  $ qemu-system-x86_64 -M nitro-enclave,vsock=c,id=hello-world \
+     -kernel hello-world.eif -nographic -m 4G --enable-kvm -cpu host \
+     -chardev socket,id=c,path=/tmp/vhost4.socket
+
+In this example, the nitro-enclave VM has CID 4.
+
+
+Limitations
+-----------
+
+AWS nitro enclave emulation support in QEMU requires users to run vhost-device-vsock
+or similar tool for vhost-user-vsock support and another VM with CID 3 with necessary
+vsock communication support. Requirement of running another VM and necessary applications
+inside it can be lifted if some proxying support is added to vhost-device-vsock to
+forward all packets to the host machine, in which case, users can run the necessary
+applications in the host machine instead of the parent VM with CID 3.
-- 
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