[DOC][PATCH] powerpc: Provide initial documentation for PAPR hcalls

Wed Aug 28 01:52:55 AEST 2019

Le 27/08/2019 à 17:23, Vaibhav Jain a écrit :
> 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 at linux.ibm.com>

Hi Vaibhav,

Thanks for documenting this.

Besides my few remarks below, please consider:

Reviewed-by: Laurent Dufour <ldufour at linux.ibm.com>

> ---
> Change-log:
> 
> Initial version of this doc-patch was posted and reviewed as part of
> the patch-series "[PATCH v5 0/4] powerpc/papr_scm: Workaround for
> failure of drc bind after kexec"
> https://patchwork.ozlabs.org/patch/1136022/. Changes introduced on top
> the original patch:
> 
> * Replaced the of term PHYP with Hypervisor to indicate both
> PowerVM/Qemu [Laurent]
> * Emphasized that In/Out arguments to hcalls are in Big-endian format
> [Laurent]
> * Fixed minor word repetition, spell issues and grammatical error
> [Michal, Mpe]
> * Replaced various variant of term 'hcall' with a single
> variant. [Mpe]
> * Changed the documentation format from txt to ReST. [Mpe]
> * Changed the name of documentation file to papr_hcalls.rst. [Mpe]
> * Updated the section describing privileged operation by hypervisor
> to be more accurate [Mpe].
> * Fixed up mention of register notation used for describing
> hcalls. [Mpe]
> * s/NVDimm/NVDIMM [Mpe]
> * Added section on return values from hcall [Mpe]
> * Described H_CONTINUE return-value for long running hcalls.
> ---
>   Documentation/powerpc/papr_hcalls.rst | 200 ++++++++++++++++++++++++++
>   1 file changed, 200 insertions(+)
>   create mode 100644 Documentation/powerpc/papr_hcalls.rst
> 
> diff --git a/Documentation/powerpc/papr_hcalls.rst b/Documentation/powerpc/papr_hcalls.rst
> new file mode 100644
> index 000000000000..7afc0310de29
> --- /dev/null
> +++ b/Documentation/powerpc/papr_hcalls.rst
> @@ -0,0 +1,200 @@
> +===========================
> +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
                  ^ hcall ?

> +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* in
> +Big-endian byte order.
Indeed, register valuer are not byte ordered, only values passed through 
buffer in memory are byte ordered.

Should it be explicitly said that Big-endian order is only concerning data 
stored in memory?
What about something like that:
"...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 are in Big-endian 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-arguments, all the
> +out value are in Big-endian byte order.
Same would apply here.

> +
> +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.
> +
> +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
> 