[PATCH v4 4/5] dt-bindings: rng: Add vmgenid support

[Sudan Landge]

Virtual Machine Generation ID driver was introduced in commit af6b54e2b5ba
("virt: vmgenid: notify RNG of VM fork and supply generation ID"), as an
ACPI only device.

VMGenID specification http://go.microsoft.com/fwlink/?LinkId=260709 defines
a mechanism for the BIOS/hypervisors to communicate to the virtual machine
that it is executed with a different configuration (e.g. snapshot execution
or creation from a template).
The guest operating system can use the notification for various purposes
such as re-initializing its random number generator etc.

As per the specs, hypervisor should provide a globally unique identified,
or GUID via ACPI.

This patch tries to mimic the mechanism to provide the same functionality
which is for a hypervisor/BIOS to notify the virtual machine when it is
executed with a different configuration.

As part of this support the devicetree bindings requires the hypervisors or
BIOS to provide a memory address which holds the GUID and an IRQ which is
used to notify when there is a change in the GUID.
The memory exposed in the DT should follow the rules defined in the
vmgenid spec mentioned above.

*Reason for this change*:
Chosing ACPI or devicetree is an intrinsic part of an hypervisor design.
Without going into details of why a hypervisor would choose DT over ACPI,
we would like to highlight that the hypervisors that have chose devicetree
and now want to make use of the vmgenid functionality cannot do so today
because vmgenid is an ACPI only device.
This forces these hypervisors to change their design which could have
undesirable impacts on their use-cases, test-scenarios etc.

vmgenid exposes to the guest a 16-byte cryptographically random number,
the value of which changes every time it starts executing from a new
configuration (snapshot, backup, etc.). During initialization, the device
exposes to the guest the address of the generation ID and
an interrupt number, which the device will use to notify the guest when
the generation ID changes.
These attributes can be trivially communicated via device tree bindings.

We believe that adding a devicetree binding for vmgenid is a simpler
alternative way to expose the device to the guest than forcing the
hypervisors to implement ACPI.

More references to vmgenid specs:
 - https://www.qemu.org/docs/master/specs/vmgenid.html
 - https://learn.microsoft.com/en-us/windows/win32/hyperv_v2/virtual-
machine-generation-identifier

Signed-off-by: Sudan Landge <sudanl@xxxxxxxxxx>
---
 .../bindings/rng/microsoft,vmgenid.yaml       | 49 +++++++++++++++++++
 MAINTAINERS                                   |  1 +
 2 files changed, 50 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml

diff --git a/Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml b/Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
new file mode 100644
index 000000000000..8f20dee93e7e
--- /dev/null
+++ b/Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rng/microsoft,vmgenid.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Virtual Machine Generation ID
+
+maintainers:
+  - Jason A. Donenfeld <Jason@xxxxxxxxx>
+
+description:
+  Firmwares or hypervisors can use this devicetree to describe an
+  interrupt and a shared resource to inject a Virtual Machine Generation ID.
+  Virtual Machine Generation ID is a globally unique identifier (GUID) and
+  the devicetree binding follows VMGenID specification defined in
+  http://go.microsoft.com/fwlink/?LinkId=260709.
+
+properties:
+  compatible:
+    const: microsoft,vmgenid
+
+  reg:
+    description:
+      Specifies a 16-byte VMGenID in endianness-agnostic hexadecimal format.
+    maxItems: 1
+
+  interrupts:
+    description:
+      Interrupt used to notify that a new VMGenID is available.
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    rng@80000000 {
+      compatible = "microsoft,vmgenid";
+      reg = <0x80000000 0x1000>;
+      interrupts = <GIC_SPI 35 IRQ_TYPE_EDGE_RISING>;
+    };
+
+...
diff --git a/MAINTAINERS b/MAINTAINERS
index aea47e04c3a5..243607744b7e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18476,6 +18476,7 @@ M:	"Theodore Ts'o" <tytso@xxxxxxx>
 M:	Jason A. Donenfeld <Jason@xxxxxxxxx>
 S:	Maintained
 T:	git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
+F:	Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
 F:	drivers/char/random.c
 F:	drivers/virt/vmgenid.c
 
-- 
2.34.1