[PATCH v10 02/12] KVM: arm64: Document KVM_ARM_GET_REG_WRITABLE_MASKS

[Oliver Upton <oliver.upton@linux.dev>]

From: Jing Zhang <jingzhangos@google.com>

Add some basic documentation on how to get feature ID register writable
masks from userspace.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
---
 Documentation/virt/kvm/api.rst | 43 ++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 21a7578142a1..8cd91bc550bc 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6070,6 +6070,49 @@ 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_FEATURE_ID_RANGES
+:Architectures: arm64
+:Type: vm ioctl
+:Parameters: struct reg_mask_range (in/out)
+:Returns: 0 on success, < 0 on error
+
+
+::
+
+        #define ARM64_FEATURE_ID_SPACE_SIZE	(3 * 8 * 8)
+        #define ARM64_FEATURE_ID_RANGE_IDREGS	BIT(0)
+
+        struct reg_mask_range {
+                __u64 addr;             /* Pointer to mask array */
+                __u32 range;            /* Requested range */
+                __u32 reserved[13];
+        };
+
+This ioctl copies the writable masks for Feature ID registers to userspace.
+The Feature ID space 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 array pointed to by ``addr`` is indexed by the macro
+``ARM64_FEATURE_ID_SPACE_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.
+
+The ``range`` field describes the requested range of registers. The valid
+ranges can be retrieved by checking the return value of
+KVM_CAP_CHECK_EXTENSION_VM for the KVM_CAP_ARM_SUPPORTED_FEATURE_ID_RANGES
+capability, which will return a bitmask of the supported ranges. Each bit
+set in the return value represents a possible value for the ``range``
+field.  At the time of writing, only bit 0 is returned set by the
+capability, meaning that only the value ``ARM64_FEATURE_ID_RANGE_IDREGS``
+is valid for ``range``.
+
+The ``reserved[13]`` array is reserved for future use and should be 0, or
+KVM may return an error.
+
 5. The kvm_run structure
 ========================
 
-- 
2.42.0.515.g380fc7ccd1-goog