[PATCH v7 02/10] KVM: arm64: Document KVM_ARM_GET_FEATURE_ID_WRITABLE_MASKS

[Jing Zhang]

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

Signed-off-by: Jing Zhang <jingzhangos at google.com>
---
 Documentation/virt/kvm/api.rst | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index c0ddd3035462..e6cda4169764 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6068,6 +6068,32 @@ 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_FEATURE_ID_WRITABLE_MASKS
+-------------------------------------------
+
+:Capability: none
+:Architectures: arm64
+:Type: vm ioctl
+:Parameters: struct feature_id_writable_masks (out)
+:Returns: 0 on success, < 0 on error
+
+
+::
+
+        #define ARM64_FEATURE_ID_SPACE_SIZE	(3 * 8 * 8)
+
+        struct feature_id_writable_masks {
+                __u64 mask[ARM64_FEATURE_ID_SPACE_SIZE];
+        };
+
+This ioctl would copy the writable masks for feature ID registers to userspace.
+The Feature ID space is defined as the System register space in AArch64 with
+op0==3, op1=={0, 1, 3}, CRn==0, CRm=={0-7}, op2=={0-7}.
+To get the index in ``mask`` array for a specified feature ID register, use the
+macro ``ARM64_FEATURE_ID_SPACE_IDX(op0, op1, crn, crm, op2)``.
+This allows the userspace to know upfront whether it can actually tweak the
+contents of a feature ID register or not.
+
 5. The kvm_run structure
 ========================
 
-- 
2.41.0.585.gd2178a4bd4-goog