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kernel-ark/Documentation/powerpc/papr_hcalls.rst
Vaibhav Jain 58b278f568 powerpc: Provide initial documentation for PAPR hcalls 
This doc patch provides an initial description of the hcall op-codes
that are used by Linux kernel running as a guest (LPAR) on top of
PowerVM or any other sPAPR compliant hyper-visor (e.g qemu).

Apart from documenting the hcalls the doc-patch also provides a
rudimentary overview of how hcall ABI, how they are issued with the
Linux kernel and how information/control flows between the guest and
hypervisor.

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Add SPDX tag, add it to index.rst]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190828082729.16695-1-vaibhav@linux.ibm.com
2020-01-29 21:30:50 +11:00

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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
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