KVM: arm64: Document KVM_ARM_GET_REG_WRITABLE_MASKS

Add some basic documentation on how to get feature ID register writable
masks from userspace.
Signed-off-by: Jing Zhang <jingzhangos@google.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231003230408.3405722-3-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

Documentation/virt/kvm/api.rst

@@ -6070,6 +6070,54 @@ writes to the CNTVCT_EL0 and CNTPCT_EL0 registers using the SET_ONE_REG
 interface. No error will be returned, but the resulting offset will not be
 applied.
 
+4.139 KVM_ARM_GET_REG_WRITABLE_MASKS
+-------------------------------------------
+
+:Capability: KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES
+:Architectures: arm64
+:Type: vm ioctl
+:Parameters: struct reg_mask_range (in/out)
+:Returns: 0 on success, < 0 on error
+
+
+::
+
+        #define KVM_ARM_FEATURE_ID_RANGE	0
+        #define KVM_ARM_FEATURE_ID_RANGE_SIZE	(3 * 8 * 8)
+
+        struct reg_mask_range {
+                __u64 addr;             /* Pointer to mask array */
+                __u32 range;            /* Requested range */
+                __u32 reserved[13];
+        };
+
+This ioctl copies the writable masks for a selected range of registers to
+userspace.
+
+The ``addr`` field is a pointer to the destination array where KVM copies
+the writable masks.
+
+The ``range`` field indicates the requested range of registers.
+``KVM_CHECK_EXTENSION`` for the ``KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES``
+capability returns the supported ranges, expressed as a set of flags. Each
+flag's bit index represents a possible value for the ``range`` field.
+All other values are reserved for future use and KVM may return an error.
+
+The ``reserved[13]`` array is reserved for future use and should be 0, or
+KVM may return an error.
+
+KVM_ARM_FEATURE_ID_RANGE (0)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The Feature ID range is defined as the AArch64 System register space with
+op0==3, op1=={0, 1, 3}, CRn==0, CRm=={0-7}, op2=={0-7}.
+
+The mask returned array pointed to by ``addr`` is indexed by the macro
+``ARM64_FEATURE_ID_RANGE_IDX(op0, op1, crn, crm, op2)``, allowing userspace
+to know what fields can be changed for the system register described by
+``op0, op1, crn, crm, op2``. KVM rejects ID register values that describe a
+superset of the features supported by the system.
+
 5. The kvm_run structure
 ========================