[PATCH v17 01/35] docs: gunyah: Introduce Gunyah Hypervisor

[Elliot Berman <quic_eberman@quicinc.com>]

Gunyah is an open-source Type-1 hypervisor developed by Qualcomm. It
does not depend on any lower-privileged OS/kernel code for its core
functionality. This increases its security and can support a smaller
trusted computing based when compared to Type-2 hypervisors.

Add documentation describing the Gunyah hypervisor and the main
components of the Gunyah hypervisor which are of interest to Linux
virtualization development.

Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
---
 Documentation/virt/gunyah/index.rst         | 135 ++++++++++++++++++++++++++++
 Documentation/virt/gunyah/message-queue.rst |  68 ++++++++++++++
 Documentation/virt/index.rst                |   1 +
 3 files changed, 204 insertions(+)

diff --git a/Documentation/virt/gunyah/index.rst b/Documentation/virt/gunyah/index.rst
new file mode 100644
index 0000000000000..fba2c7a11d0f8
--- /dev/null
+++ b/Documentation/virt/gunyah/index.rst
@@ -0,0 +1,135 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Gunyah Hypervisor
+=================
+
+.. toctree::
+   :maxdepth: 1
+
+   message-queue
+
+Gunyah is a Type-1 hypervisor which is independent of any OS kernel, and runs in
+a more privileged CPU level (EL2 on Aarch64). It does not depend on a less
+privileged operating system for its core functionality. This increases its
+security and can support a much smaller trusted computing base than a Type-2
+hypervisor.
+
+Gunyah is an open source hypervisor. The source repository is available at
+https://github.com/quic/gunyah-hypervisor.
+
+Gunyah provides these following features.
+
+- Scheduling:
+
+  A scheduler for virtual CPUs (vCPUs) on physical CPUs enables time-sharing
+  of the CPUs. Gunyah supports two models of scheduling which can coexist on
+  a running system:
+
+    1. Hypervisor vCPU scheduling in which Gunyah hypervisor schedules vCPUS on
+       its own. The default is a real-time priority with round-robin scheduler.
+    2. "Proxy" scheduling in which an owner-VM can donate the remainder of its
+       own vCPU's time slice to an owned-VM's vCPU via a hypercall.
+
+- Memory Management:
+
+  APIs handling memory, abstracted as objects, limiting direct use of physical
+  addresses. Memory ownership and usage tracking of all memory under its control.
+  Memory partitioning between VMs is a fundamental security feature.
+
+- Interrupt Virtualization:
+
+  Interrupt ownership is tracked and interrupt delivery is directly to the
+  assigned VM. Gunyah makes use of hardware interrupt virtualization where
+  possible.
+
+- Inter-VM Communication:
+
+  There are several different mechanisms provided for communicating between VMs.
+
+    1. Message queues
+    2. Doorbells
+    3. Virtio MMIO transport
+    4. Shared memory
+
+- Virtual platform:
+
+  Architectural devices such as interrupt controllers and CPU timers are
+  directly provided by the hypervisor as well as core virtual platform devices
+  and system APIs such as ARM PSCI.
+
+- Device Virtualization:
+
+  Para-virtualization of devices is supported using inter-VM communication and
+  virtio transport support. Select stage 2 faults by virtual machines that use
+  proxy-scheduled vCPUs can be handled directly by Linux to provide Type-2
+  hypervisor style on-demand paging and/or device emulation.
+
+Architectures supported
+=======================
+AArch64 with a GICv3 or GICv4.1
+
+Resources and Capabilities
+==========================
+
+Services/resources provided by the Gunyah hypervisor are accessible to a
+virtual machine through capabilities. A capability is an access control
+token granting the holder a set of permissions to operate on a specific
+hypervisor object (conceptually similar to a file-descriptor).
+For example, inter-VM communication using Gunyah doorbells and message queues
+is performed using hypercalls taking Capability ID arguments for the required
+IPC objects. These resources are described in Linux as a struct gunyah_resource.
+
+Unlike UNIX file descriptors, there is no path-based or similar lookup of
+an object to create a new Capability, meaning simpler security analysis.
+Creation of a new Capability requires the holding of a set of privileged
+Capabilities which are typically never given out by the Resource Manager (RM).
+
+Gunyah itself provides no APIs for Capability ID discovery. Enumeration of
+Capability IDs is provided by RM as a higher level service to VMs.
+
+Resource Manager
+================
+
+The Gunyah Resource Manager (RM) is a privileged application VM supporting the
+Gunyah Hypervisor. It provides policy enforcement aspects of the virtualization
+system. The resource manager can be treated as an extension of the Hypervisor
+but is separated to its own partition to ensure that the hypervisor layer itself
+remains small and secure and to maintain a separation of policy and mechanism in
+the platform. The resource manager runs at arm64 NS-EL1, similar to other
+virtual machines.
+
+Communication with the resource manager from other virtual machines happens as
+described in message-queue.rst. Details about the specific messages can be found
+in drivers/virt/gunyah/rsc_mgr.c
+
+::
+
+  +-------+   +--------+   +--------+
+  |  RM   |   |  VM_A  |   |  VM_B  |
+  +-.-.-.-+   +---.----+   +---.----+
+    | |           |            |
+  +-.-.-----------.------------.----+
+  | | \==========/             |    |
+  |  \========================/     |
+  |            Gunyah               |
+  +---------------------------------+
+
+The source for the resource manager is available at
+https://github.com/quic/gunyah-resource-manager.
+
+The resource manager provides the following features:
+
+- VM lifecycle management: allocating a VM, starting VMs, destruction of VMs
+- VM access control policy, including memory sharing and lending
+- Interrupt routing configuration
+- Forwarding of system-level events (e.g. VM shutdown) to owner VM
+- Resource (capability) discovery
+
+A VM requires boot configuration to establish communication with the resource
+manager. This is provided to VMs via a 'hypervisor' device tree node which is
+overlaid to the VMs DT by the RM. This node lets guests know they are running
+as a Gunyah guest VM, how to communicate with resource manager, and basic
+description and capabilities of this VM. See
+Documentation/devicetree/bindings/firmware/gunyah-hypervisor.yaml for a
+description of this node.
diff --git a/Documentation/virt/gunyah/message-queue.rst b/Documentation/virt/gunyah/message-queue.rst
new file mode 100644
index 0000000000000..96864708f442e
--- /dev/null
+++ b/Documentation/virt/gunyah/message-queue.rst
@@ -0,0 +1,68 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Message Queues
+==============
+Message queue is a simple low-capacity IPC channel between two virtual machines.
+It is intended for sending small control and configuration messages. Each
+message queue is unidirectional and buffered in the hypervisor. A full-duplex
+IPC channel requires a pair of queues.
+
+The size of the queue and the maximum size of the message that can be passed is
+fixed at creation of the message queue. Resource manager is presently the only
+use case for message queues, and creates messages queues between itself and VMs
+with a fixed maximum message size of 240 bytes. Longer messages require a
+further protocol on top of the message queue messages themselves. For instance,
+communication with the resource manager adds a header field for sending longer
+messages which are split into smaller fragments.
+
+The diagram below shows how message queue works. A typical configuration
+involves 2 message queues. Message queue 1 allows VM_A to send messages to VM_B.
+Message queue 2 allows VM_B to send messages to VM_A.
+
+1. VM_A sends a message of up to 240 bytes in length. It makes a hypercall
+   with the message to request the hypervisor to add the message to
+   message queue 1's queue. The hypervisor copies memory into the internal
+   message queue buffer; the memory doesn't need to be shared between
+   VM_A and VM_B.
+
+2. Gunyah raises the corresponding interrupt for VM_B (Rx vIRQ) when any of
+   these happens:
+
+   a. gunyah_msgq_send() has PUSH flag. This is a typical case when the message
+      queue is being used to implement an RPC-like interface.
+   b. Explicitly with gunyah_msgq_push hypercall from VM_A.
+   c. Message queue has reached a threshold depth. Typically, this threshold
+      depth is the size of the queue (in other words: when queue is full, Rx
+      vIRQ is raised).
+
+3. VM_B calls gunyah_msgq_recv() and Gunyah copies message to requested buffer.
+
+4. Gunyah raises the corresponding interrupt for VM_A (Tx vIRQ) when the message
+   queue falls below a watermark depth. Typically, this is when the queue is
+   drained. Note the watermark depth and the threshold depth for the Rx vIRQ are
+   independent values. Coincidentally, this signal is conceptually similar to
+   Clear-to-Send.
+
+For VM_B to send a message to VM_A, the process is identical, except that
+hypercalls reference message queue 2's capability ID. The IRQ will be different
+for the second message queue.
+
+::
+
+      +-------------------+         +-----------------+         +-------------------+
+      |        VM_A       |         |Gunyah hypervisor|         |        VM_B       |
+      |                   |         |                 |         |                   |
+      |                   |         |                 |         |                   |
+      |                   |   Tx    |                 |         |                   |
+      |                   |-------->|                 | Rx vIRQ |                   |
+      |gunyah_msgq_send() | Tx vIRQ |Message queue 1  |-------->|gunyah_msgq_recv() |
+      |                   |<------- |                 |         |                   |
+      |                   |         |                 |         |                   |
+      |                   |         |                 |         |                   |
+      |                   |         |                 |   Tx    |                   |
+      |                   | Rx vIRQ |                 |<--------|                   |
+      |gunyah_msgq_recv() |<--------|Message queue 2  | Tx vIRQ |gunyah_msgq_send() |
+      |                   |         |                 |-------->|                   |
+      |                   |         |                 |         |                   |
+      |                   |         |                 |         |                   |
+      +-------------------+         +-----------------+         +---------------+
diff --git a/Documentation/virt/index.rst b/Documentation/virt/index.rst
index 7fb55ae08598d..15869ee059b35 100644
--- a/Documentation/virt/index.rst
+++ b/Documentation/virt/index.rst
@@ -16,6 +16,7 @@ Virtualization Support
    coco/sev-guest
    coco/tdx-guest
    hyperv/index
+   gunyah/index
 
 .. only:: html and subproject
 

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