.. SPDX-License-Identifier: GPL-2.0

Paravirtualized sched support for arm64
=======================================

KVM/arm64 provides some hypervisor service calls to support a paravirtualized
sched.

Some SMCCC compatible hypercalls are defined:

* PV_SCHED_FEATURES:          0xC5000090
* PV_SCHED_IPA_INIT:          0xC5000091
* PV_SCHED_IPA_RELEASE:       0xC5000092

The existence of the PV_SCHED hypercall should be probed using the SMCCC 1.1
ARCH_FEATURES mechanism before calling it.

PV_SCHED_FEATURES
    ============= ========    ==========
    Function ID:  (uint32)    0xC5000090
    PV_call_id:   (uint32)    The function to query for support.
    Return value: (int64)     NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant
                              PV-sched feature is supported by the hypervisor.
    ============= ========    ==========

PV_SCHED_IPA_INIT
    ============= ========    ==========
    Function ID:  (uint32)    0xC5000091
    Return value: (int64)     NOT_SUPPORTED (-1) or SUCCESS (0) if the IPA of
                              this vCPU's PV data structure is shared to the
                              hypervisor.
    ============= ========    ==========

PV_SCHED_IPA_RELEASE
    ============= ========    ==========
    Function ID:  (uint32)    0xC5000092
    Return value: (int64)     NOT_SUPPORTED (-1) or SUCCESS (0) if the IPA of
                              this vCPU's PV data structure is released.
    ============= ========    ==========

PV sched state
--------------

The structure pointed to by the PV_SCHED_IPA hypercall is as follows:

+-----------+-------------+-------------+-----------------------------------+
| Field     | Byte Length | Byte Offset | Description                       |
+===========+=============+=============+===================================+
| preempted |      4      |      0      | Indicates that the vCPU that owns |
|           |             |             | this struct is running or not.    |
|           |             |             | Non-zero values mean the vCPU has |
|           |             |             | been preempted. Zero means the    |
|           |             |             | vCPU is not preempted.            |
+-----------+-------------+-------------+-----------------------------------+

The preempted field will be updated to 0 by the hypervisor prior to scheduling
a vCPU. When the vCPU is scheduled out, the preempted field will be updated
to 1 by the hypervisor.