.. SPDX-License-Identifier: GPL-2.0
  
  ===========================
  Hypercall Op-codes (hcalls)
  ===========================
  
  Overview
  =========
  
  Virtualization on 64-bit Power Book3S Platforms is based on the PAPR
  specification [1]_ which describes the run-time environment for a guest
  operating system and how it should interact with the hypervisor for
  privileged operations. Currently there are two PAPR compliant hypervisors:
  
  - **IBM PowerVM (PHYP)**: IBM's proprietary hypervisor that supports AIX,
    IBM-i and  Linux as supported guests (termed as Logical Partitions
    or LPARS). It supports the full PAPR specification.
  
  - **Qemu/KVM**: Supports PPC64 linux guests running on a PPC64 linux host.
    Though it only implements a subset of PAPR specification called LoPAPR [2]_.
  
  On PPC64 arch a guest kernel running on top of a PAPR hypervisor is called
  a *pSeries guest*. A pseries guest runs in a supervisor mode (HV=0) and must
  issue hypercalls to the hypervisor whenever it needs to perform an action
  that is hypervisor priviledged [3]_ or for other services managed by the
  hypervisor.
  
  Hence a Hypercall (hcall) is essentially a request by the pseries guest
  asking hypervisor to perform a privileged operation on behalf of the guest. The
  guest issues a with necessary input operands. The hypervisor after performing
  the privilege operation returns a status code and output operands back to the
  guest.
  
  HCALL ABI
  =========
  The ABI specification for a hcall between a pseries guest and PAPR hypervisor
  is covered in section 14.5.3 of ref [2]_. Switch to the  Hypervisor context is
  done via the instruction **HVCS** that expects the Opcode for hcall is set in *r3*
  and any in-arguments for the hcall are provided in registers *r4-r12*. If values
  have to be passed through a memory buffer, the data stored in that buffer should be
  in Big-endian byte order.
  
  Once control is returns back to the guest after hypervisor has serviced the
  'HVCS' instruction the return value of the hcall is available in *r3* and any
  out values are returned in registers *r4-r12*. Again like in case of in-arguments,
  any out values stored in a memory buffer will be in Big-endian byte order.
  
  Powerpc arch code provides convenient wrappers named **plpar_hcall_xxx** defined
  in a arch specific header [4]_ to issue hcalls from the linux kernel
  running as pseries guest.
  
  Register Conventions
  ====================
  
  Any hcall should follow same register convention as described in section 2.2.1.1
  of "64-Bit ELF V2 ABI Specification: Power Architecture"[5]_. Table below
  summarizes these conventions:
  
  +----------+----------+-------------------------------------------+
  | Register |Volatile  |  Purpose                                  |
  | Range    |(Y/N)     |                                           |
  +==========+==========+===========================================+
  |   r0     |    Y     |  Optional-usage                           |
  +----------+----------+-------------------------------------------+
  |   r1     |    N     |  Stack Pointer                            |
  +----------+----------+-------------------------------------------+
  |   r2     |    N     |  TOC                                      |
  +----------+----------+-------------------------------------------+
  |   r3     |    Y     |  hcall opcode/return value                |
  +----------+----------+-------------------------------------------+
  |  r4-r10  |    Y     |  in and out values                        |
  +----------+----------+-------------------------------------------+
  |   r11    |    Y     |  Optional-usage/Environmental pointer     |
  +----------+----------+-------------------------------------------+
  |   r12    |    Y     |  Optional-usage/Function entry address at |
  |          |          |  global entry point                       |
  +----------+----------+-------------------------------------------+
  |   r13    |    N     |  Thread-Pointer                           |
  +----------+----------+-------------------------------------------+
  |  r14-r31 |    N     |  Local Variables                          |
  +----------+----------+-------------------------------------------+
  |    LR    |    Y     |  Link Register                            |
  +----------+----------+-------------------------------------------+
  |   CTR    |    Y     |  Loop Counter                             |
  +----------+----------+-------------------------------------------+
  |   XER    |    Y     |  Fixed-point exception register.          |
  +----------+----------+-------------------------------------------+
  |  CR0-1   |    Y     |  Condition register fields.               |
  +----------+----------+-------------------------------------------+
  |  CR2-4   |    N     |  Condition register fields.               |
  +----------+----------+-------------------------------------------+
  |  CR5-7   |    Y     |  Condition register fields.               |
  +----------+----------+-------------------------------------------+
  |  Others  |    N     |                                           |
  +----------+----------+-------------------------------------------+
  
  DRC & DRC Indexes
  =================
  ::
  
       DR1                                  Guest
       +--+        +------------+         +---------+
       |  | <----> |            |         |  User   |
       +--+  DRC1  |            |   DRC   |  Space  |
                   |    PAPR    |  Index  +---------+
       DR2         | Hypervisor |         |         |
       +--+        |            | <-----> |  Kernel |
       |  | <----> |            |  Hcall  |         |
       +--+  DRC2  +------------+         +---------+
  
  PAPR hypervisor terms shared hardware resources like PCI devices, NVDIMMs etc
  available for use by LPARs as Dynamic Resource (DR). When a DR is allocated to
  an LPAR, PHYP creates a data-structure called Dynamic Resource Connector (DRC)
  to manage LPAR access. An LPAR refers to a DRC via an opaque 32-bit number
  called DRC-Index. The DRC-index value is provided to the LPAR via device-tree
  where its present as an attribute in the device tree node associated with the
  DR.
  
  HCALL Return-values
  ===================
  
  After servicing the hcall, hypervisor sets the return-value in *r3* indicating
  success or failure of the hcall. In case of a failure an error code indicates
  the cause for error. These codes are defined and documented in arch specific
  header [4]_.
  
  In some cases a hcall can potentially take a long time and need to be issued
  multiple times in order to be completely serviced. These hcalls will usually
  accept an opaque value *continue-token* within there argument list and a
  return value of *H_CONTINUE* indicates that hypervisor hasn't still finished
  servicing the hcall yet.
  
  To make such hcalls the guest need to set *continue-token == 0* for the
  initial call and use the hypervisor returned value of *continue-token*
  for each subsequent hcall until hypervisor returns a non *H_CONTINUE*
  return value.
  
  HCALL Op-codes
  ==============
  
  Below is a partial list of HCALLs that are supported by PHYP. For the
  corresponding opcode values please look into the arch specific header [4]_:
  
  **H_SCM_READ_METADATA**
  
  | Input: *drcIndex, offset, buffer-address, numBytesToRead*
  | Out: *numBytesRead*
  | Return Value: *H_Success, H_Parameter, H_P2, H_P3, H_Hardware*
  
  Given a DRC Index of an NVDIMM, read N-bytes from the the metadata area
  associated with it, at a specified offset and copy it to provided buffer.
  The metadata area stores configuration information such as label information,
  bad-blocks etc. The metadata area is located out-of-band of NVDIMM storage
  area hence a separate access semantics is provided.
  
  **H_SCM_WRITE_METADATA**
  
  | Input: *drcIndex, offset, data, numBytesToWrite*
  | Out: *None*
  | Return Value: *H_Success, H_Parameter, H_P2, H_P4, H_Hardware*
  
  Given a DRC Index of an NVDIMM, write N-bytes to the metadata area
  associated with it, at the specified offset and from the provided buffer.
  
  **H_SCM_BIND_MEM**
  
  | Input: *drcIndex, startingScmBlockIndex, numScmBlocksToBind,*
  | *targetLogicalMemoryAddress, continue-token*
  | Out: *continue-token, targetLogicalMemoryAddress, numScmBlocksToBound*
  | Return Value: *H_Success, H_Parameter, H_P2, H_P3, H_P4, H_Overlap,*
  | *H_Too_Big, H_P5, H_Busy*
  
  Given a DRC-Index of an NVDIMM, map a continuous SCM blocks range
  *(startingScmBlockIndex, startingScmBlockIndex+numScmBlocksToBind)* to the guest
  at *targetLogicalMemoryAddress* within guest physical address space. In
  case *targetLogicalMemoryAddress == 0xFFFFFFFF_FFFFFFFF* then hypervisor
  assigns a target address to the guest. The HCALL can fail if the Guest has
  an active PTE entry to the SCM block being bound.
  
  **H_SCM_UNBIND_MEM**
  | Input: drcIndex, startingScmLogicalMemoryAddress, numScmBlocksToUnbind
  | Out: numScmBlocksUnbound
  | Return Value: *H_Success, H_Parameter, H_P2, H_P3, H_In_Use, H_Overlap,*
  | *H_Busy, H_LongBusyOrder1mSec, H_LongBusyOrder10mSec*
  
  Given a DRC-Index of an NVDimm, unmap *numScmBlocksToUnbind* SCM blocks starting
  at *startingScmLogicalMemoryAddress* from guest physical address space. The
  HCALL can fail if the Guest has an active PTE entry to the SCM block being
  unbound.
  
  **H_SCM_QUERY_BLOCK_MEM_BINDING**
  
  | Input: *drcIndex, scmBlockIndex*
  | Out: *Guest-Physical-Address*
  | Return Value: *H_Success, H_Parameter, H_P2, H_NotFound*
  
  Given a DRC-Index and an SCM Block index return the guest physical address to
  which the SCM block is mapped to.
  
  **H_SCM_QUERY_LOGICAL_MEM_BINDING**
  
  | Input: *Guest-Physical-Address*
  | Out: *drcIndex, scmBlockIndex*
  | Return Value: *H_Success, H_Parameter, H_P2, H_NotFound*
  
  Given a guest physical address return which DRC Index and SCM block is mapped
  to that address.
  
  **H_SCM_UNBIND_ALL**
  
  | Input: *scmTargetScope, drcIndex*
  | Out: *None*
  | Return Value: *H_Success, H_Parameter, H_P2, H_P3, H_In_Use, H_Busy,*
  | *H_LongBusyOrder1mSec, H_LongBusyOrder10mSec*
  
  Depending on the Target scope unmap all SCM blocks belonging to all NVDIMMs
  or all SCM blocks belonging to a single NVDIMM identified by its drcIndex
  from the LPAR memory.
  
  **H_SCM_HEALTH**
  
  | Input: drcIndex

  | Out: *health-bitmap (r4), health-bit-valid-bitmap (r5)*

  | Return Value: *H_Success, H_Parameter, H_Hardware*
  
  Given a DRC Index return the info on predictive failure and overall health of

  the PMEM device. The asserted bits in the health-bitmap indicate one or more states
  (described in table below) of the PMEM device and health-bit-valid-bitmap indicate
  which bits in health-bitmap are valid. The bits are reported in
  reverse bit ordering for example a value of 0xC400000000000000
  indicates bits 0, 1, and 5 are valid.
  
  Health Bitmap Flags:
  
  +------+-----------------------------------------------------------------------+
  |  Bit |               Definition                                              |
  +======+=======================================================================+
  |  00  | PMEM device is unable to persist memory contents.                     |
  |      | If the system is powered down, nothing will be saved.                 |
  +------+-----------------------------------------------------------------------+
  |  01  | PMEM device failed to persist memory contents. Either contents were   |
  |      | not saved successfully on power down or were not restored properly on |
  |      | power up.                                                             |
  +------+-----------------------------------------------------------------------+
  |  02  | PMEM device contents are persisted from previous IPL. The data from   |
  |      | the last boot were successfully restored.                             |
  +------+-----------------------------------------------------------------------+
  |  03  | PMEM device contents are not persisted from previous IPL. There was no|
  |      | data to restore from the last boot.                                   |
  +------+-----------------------------------------------------------------------+
  |  04  | PMEM device memory life remaining is critically low                   |
  +------+-----------------------------------------------------------------------+
  |  05  | PMEM device will be garded off next IPL due to failure                |
  +------+-----------------------------------------------------------------------+
  |  06  | PMEM device contents cannot persist due to current platform health    |
  |      | status. A hardware failure may prevent data from being saved or       |
  |      | restored.                                                             |
  +------+-----------------------------------------------------------------------+
  |  07  | PMEM device is unable to persist memory contents in certain conditions|
  +------+-----------------------------------------------------------------------+
  |  08  | PMEM device is encrypted                                              |
  +------+-----------------------------------------------------------------------+
  |  09  | PMEM device has successfully completed a requested erase or secure    |
  |      | erase procedure.                                                      |
  +------+-----------------------------------------------------------------------+
  |10:63 | Reserved / Unused                                                     |
  +------+-----------------------------------------------------------------------+

  
  **H_SCM_PERFORMANCE_STATS**
  
  | Input: drcIndex, resultBuffer Addr
  | Out: None
  | Return Value:  *H_Success, H_Parameter, H_Unsupported, H_Hardware, H_Authority, H_Privilege*
  
  Given a DRC Index collect the performance statistics for NVDIMM and copy them
  to the resultBuffer.
  
  References
  ==========
  .. [1] "Power Architecture Platform Reference"
         https://en.wikipedia.org/wiki/Power_Architecture_Platform_Reference
  .. [2] "Linux on Power Architecture Platform Reference"
         https://members.openpowerfoundation.org/document/dl/469
  .. [3] "Definitions and Notation" Book III-Section 14.5.3
         https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0
  .. [4] arch/powerpc/include/asm/hvcall.h
  .. [5] "64-Bit ELF V2 ABI Specification: Power Architecture"
         https://openpowerfoundation.org/?resource_lib=64-bit-elf-v2-abi-specification-power-architecture