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+.. 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, health-bit-valid-bitmap*
+| Return Value: *H_Success, H_Parameter, H_Hardware*
+
+Given a DRC Index return the info on predictive failure and overall health of
+the NVDIMM. The asserted bits in the health-bitmap indicate a single predictive
+failure and health-bit-valid-bitmap indicate which bits in health-bitmap are
+valid.
+
+**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