KVM: arm64: Document the KVM/arm64-specific calls in hypercalls.rst

KVM/arm64 makes use of the SMCCC "Vendor Specific Hypervisor Service
Call Range" to expose KVM-specific hypercalls to guests in a
discoverable and extensible fashion.

Document the existence of this interface and the discovery hypercall.

Signed-off-by: Will Deacon <[e-mail]>
Signed-off-by: Fuad Tabba <[e-mail]>
Acked-by: Oliver Upton <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Marc Zyngier <[e-mail]>

Documentation/virt/kvm/arm/hypercalls.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+===============================================
+KVM/arm64-specific hypercalls exposed to guests
+===============================================
+
+This file documents the KVM/arm64-specific hypercalls which may be
+exposed by KVM/arm64 to guest operating systems. These hypercalls are
+issued using the HVC instruction according to version 1.1 of the Arm SMC
+Calling Convention (DEN0028/C):
+
+https://developer.arm.com/docs/den0028/c
+
+All KVM/arm64-specific hypercalls are allocated within the "Vendor
+Specific Hypervisor Service Call" range with a UID of
+``28b46fb6-2ec5-11e9-a9ca-4b564d003a74``. This UID should be queried by the
+guest using the standard "Call UID" function for the service range in
+order to determine that the KVM/arm64-specific hypercalls are available.
+
+``ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID``
+---------------------------------------------
+
+Provides a discovery mechanism for other KVM/arm64 hypercalls.
+
++---------------------+-------------------------------------------------------------+
+| Presence:           | Mandatory for the KVM/arm64 UID                             |
++---------------------+-------------------------------------------------------------+
+| Calling convention: | HVC32                                                       |
++---------------------+----------+--------------------------------------------------+
+| Function ID:        | (uint32) | 0x86000000                                       |
++---------------------+----------+--------------------------------------------------+
+| Arguments:          | None                                                        |
++---------------------+----------+----+---------------------------------------------+
+| Return Values:      | (uint32) | R0 | Bitmap of available function numbers 0-31   |
+|                     +----------+----+---------------------------------------------+
+|                     | (uint32) | R1 | Bitmap of available function numbers 32-63  |
+|                     +----------+----+---------------------------------------------+
+|                     | (uint32) | R2 | Bitmap of available function numbers 64-95  |
+|                     +----------+----+---------------------------------------------+
+|                     | (uint32) | R3 | Bitmap of available function numbers 96-127 |
++---------------------+----------+----+---------------------------------------------+
+
+``ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID``
+----------------------------------------
+
+See ptp_kvm.rst

Documentation/virt/kvm/arm/index.rst

···
 
    fw-pseudo-registers
    hyp-abi
+   hypercalls
    pvtime
    ptp_kvm
    vcpu-features