[PATCH v9 02/11] KVM: arm64: Document KVM_ARM_GET_REG_WRITABLE_MASKS

[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 | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index c0ddd3035462..92a9b20f970e 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6068,6 +6068,35 @@ 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: none
+: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)
+
+        struct reg_mask_range {
+                __u64 addr;             /* Pointer to mask array */
+                __u64 reserved[7];
+        };
+
+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 the mask array pointed by ``addr`` 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.
+The ``reserved[7]`` is reserved for future use to add other register space. For
+feature ID registers, it should be 0, otherwise, KVM may return error.
+
 5. The kvm_run structure
 ========================
 
-- 
2.42.0.rc1.204.g551eb34607-goog


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