Re: [PATCH v8 01/28] docs: gunyah: Introduce Gunyah Hypervisor

From: Elliot Berman
Date: Tue Jan 10 2023 - 12:55:29 EST

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In reply to: Alex Elder: "Re: [PATCH v8 01/28] docs: gunyah: Introduce Gunyah Hypervisor"
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On 1/9/2023 1:34 PM, Alex Elder wrote:

On 12/19/22 4:58 PM, Elliot Berman wrote:

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.

Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
---
Documentation/virt/gunyah/index.rst         | 114 ++++++++++++++++++++
Documentation/virt/gunyah/message-queue.rst | 56 ++++++++++
Documentation/virt/index.rst                |   1 +
MAINTAINERS                                 |   7 ++
4 files changed, 178 insertions(+)
create mode 100644 Documentation/virt/gunyah/index.rst
create mode 100644 Documentation/virt/gunyah/message-queue.rst

diff --git a/Documentation/virt/gunyah/index.rst b/Documentation/virt/gunyah/index.rst
new file mode 100644
index 000000000000..fbadbdd24da7
--- /dev/null
+++ b/Documentation/virt/gunyah/index.rst
@@ -0,0 +1,114 @@
+.. 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 higher CPU privilege level. It does not depend on any lower-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 repo is available at
+https://github.com/quic/gunyah-hypervisor.
+
+Gunyah provides these following features.
+
+- Scheduling:
+
+ A scheduler for virtual CPUs (vCPUs) on physical CPUs and enables time-sharing

s/and enables/enables/ (?)

+ of the CPUs. Gunyah supports two models of scheduling:
+
+    1. "Behind the back" scheduling in which Gunyah hypervisor schedules vCPUS on its own.
+    2. "Proxy" scheduling in which a delegated VM can donate part of one of its vCPU slice
+       to another VM's vCPU via a hypercall.
+
+- Memory Management:
+
+ APIs handling memory, abstracted as objects, limiting direct useof physical
+ addresses. Memory ownership and usage tracking of all memory under its control.
+ Memory partitioning between VMs is a fundamental security feature.
+
+- Interrupt Virtualization:
+
+ Uses CPU hardware interrupt virtualization capabilities. Interrupts are handled
+ in the hypervisor and routed to the assigned VM.
+
+- Inter-VM Communication:
+
+ There are several different mechanisms provided for communicating between VMs.
+
+- 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.
+
+Architectures supported
+=======================
+AArch64 with a GIC
+
+Resources and Capabilities
+==========================
+
+Some services or resources provided by the Gunyah hypervisor are described to a virtual machine by
+capability IDs. For instance, inter-VM communication is performed with doorbells and message queues.
+Gunyah allows access to manipulate that doorbell via the capability ID. These devices are described

s/devices/resources/

+in Linux as a struct gunyah_resource.
+
+High level management of these resources is performed by the resource manager VM. RM informs a

s/resource manager VM/resource manager VM (RM)/

+guest VM about resources it can access through either the device tree or via guest-initiated RPC.
+
+For each virtual machine, Gunyah maintains a table of resources which can be accessed by that VM.
+An entry in this table is called a "capability" and VMs can only access resources via this
+capability table. Hence, virtual Gunyah devices are referenced by a "capability IDs" and not a

s/devices/resources/
s/and not a/and not/

+"resource IDs". A VM can have multiple capability IDs mapping to the same resource. If 2 VMs have
+access to the same resource, they may not be using the same capability ID to access that resource

Does "may not be using the same capability ID" mean they "shall not",
or "are permitted not to"?

"are permitted not to". I'll say "might not" instead of "may not".

+since the tables are independent per VM.
+
+Resource Manager
+================
+
+The 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. On arm64, RM runs at NS-EL1 similar to other virtual machines.

This only runs on arm64, right? Maybe "RM runs at arm64 NS-EL1..."

+
+Communication with the resource manager from each guest VM happens with message-queue.rst. Details

Is "message-queue.rst" supposed to be a reference to that other document?

Yes. Sphinx will generate hyperlink to that document. It's in the same directory as this document.

+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
+
+When booting a virtual machine which uses a devicetree, resource manager overlays a

"When booting Linux in a virtual machine..." ?

+/hypervisor node. This node can let Linux know it is 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 000000000000..be4ab289236a
--- /dev/null
+++ b/Documentation/virt/gunyah/message-queue.rst
@@ -0,0 +1,56 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Message Queues
+==============
+Message queue is a simple low-capacity IPC channel between two VMs. It is
+intended for sending small control and configuration messages. Each message
+queue object is unidirectional, so a full-duplex IPC channel requires a pair of
+objects.

The wording here makes it seem like "message queue" might be
distinct from a "message queue object" but I think they're the
same thing (right?).

Yes, they are the same. Removed the "object" to make it more concise.

+
+Messages can be up to 240 bytes in length. 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 via multiple
+message 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 raises a hypercall
+ with the message to inform the hypervisor to add the message to
+ message queue 1's queue.
+
+2. Gunyah raises the corresponding interrupt for VM_B when any of these happens:

(edited above line to explicitly call out this is the Rx vIRQ)

+
+   a. gh_msgq_send has PUSH flag. Queue is immediately flushed. This is the typical case.
+   b. Explicility with gh_msgq_push command from VM_A.
+   c. Message queue has reached a threshold depth.
+
+3. VM_B calls gh_msgq_recv and Gunyah copies message to requested buffer.

So VM_B *responds* to the Rx vIRQ by calling gh_msgq_recv() and
supplying a buffer in which Gunyah copies the message content?

I guess my point is, can VM_B post a receive buffer in advance of
a message Rx vIRQ being delivered?

Yes, that is possible.

You don't describe what a Tx vIRQ does. When does it fire?

Good catch! I've added a 4th point:

4. Gunyah buffers messages in the queue. If the queue became full when VM_A added a message,
the return values for gh_msgq_send() include a flag that indicates the queue is full.
Once VM_B receives the message and, thus, there is space in the queue, Gunyah
will raise the Tx vIRQ on VM_A to indicate it can continue sending messages.

Thanks,
Elliot

Next message: Elliot Berman: "Re: [PATCH v8 03/28] gunyah: Common types and error codes for Gunyah hypercalls"
Previous message: Bjorn Andersson: "Re: [PATCH V7 2/2] firmware: qcom: scm: Add wait-queue handling logic"
In reply to: Alex Elder: "Re: [PATCH v8 01/28] docs: gunyah: Introduce Gunyah Hypervisor"
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