Documentation: KVM: Introduce documentation for VGICv5

Now that it is possible to create a VGICv5 device, provide initial
documentation for it. At this stage, there is little to document.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20260319154937.3619520-39-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Documentation/virt/kvm/devices/arm-vgic-v5.rst

@@ -0,0 +1,37 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====================================================
+ARM Virtual Generic Interrupt Controller v5 (VGICv5)
+====================================================
+
+
+Device types supported:
+  - KVM_DEV_TYPE_ARM_VGIC_V5     ARM Generic Interrupt Controller v5.0
+
+Only one VGIC instance may be instantiated through this API.  The created VGIC
+will act as the VM interrupt controller, requiring emulated user-space devices
+to inject interrupts to the VGIC instead of directly to CPUs.
+
+Creating a guest GICv5 device requires a host GICv5 host.  The current VGICv5
+device only supports PPI interrupts.  These can either be injected from emulated
+in-kernel devices (such as the Arch Timer, or PMU), or via the KVM_IRQ_LINE
+ioctl.
+
+Groups:
+  KVM_DEV_ARM_VGIC_GRP_CTRL
+   Attributes:
+
+    KVM_DEV_ARM_VGIC_CTRL_INIT
+      request the initialization of the VGIC, no additional parameter in
+      kvm_device_attr.addr. Must be called after all VCPUs have been created.
+
+  Errors:
+
+    =======  ========================================================
+    -ENXIO   VGIC not properly configured as required prior to calling
+             this attribute
+    -ENODEV  no online VCPU
+    -ENOMEM  memory shortage when allocating vgic internal data
+    -EFAULT  Invalid guest ram access
+    -EBUSY   One or more VCPUS are running
+    =======  ========================================================