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 ===========================
 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 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 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.
 
 **H_SCM_FLUSH**
 
 | Input: *drcIndex, continue-token*
 | Out: *continue-token*
 | Return Value: *H_SUCCESS, H_Parameter, H_P2, H_BUSY*
 
 Given a DRC Index Flush the data to backend NVDIMM device.
 
 The hcall returns H_BUSY when the flush takes longer time and the hcall needs
 to be issued multiple times in order to be completely serviced. The
 *continue-token* from the output to be passed in the argument list of
 subsequent hcalls to the hypervisor until the hcall is completely serviced
 at which point H_SUCCESS or other error is returned by the hypervisor.
 
 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

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