Hypervisor Maintenance Interrupt (HMI)
======================================

Hypervisor Maintenance Interrupt usually reports error related to processor
recovery/checkstop, NX/NPU checkstop and Timer facility. Hypervisor then
takes this opportunity to analyze and recover from some of these errors.
Hypervisor takes assistance from OPAL layer to handle and recover from HMI.
After handling HMI, OPAL layer sends the summary of error report and status
of recovery action using HMI event. See ref:`opal-messages` for HMI
event structure under :ref:`OPAL_MSG_HMI_EVT` section.

HMI is thread specific. The reason for HMI is available in a per thread
Hypervisor Maintenance Exception Register (HMER). A Hypervisor Maintenance
Exception Enable Register (HMEER) is per core. Bits from the HMER need to
be enabled by the corresponding bits in the HMEER in order to cause an HMI.

Several interrupt reasons are routed in parallel to each of the thread
specific copies. Each thread can only clear bits in its own HMER. OPAL
handler from each thread clears the respective bit from HMER register
after handling the error.

List of errors that causes HMI
==============================

 - CPU Errors

   - Processor Core checkstop
   - Processor retry recovery
   - NX/NPU/CAPP checkstop.

 - Timer facility Errors

   - ChipTOD Errors

    - ChipTOD sync check and parity errors
    - ChipTOD configuration register parity errors
    - ChiTOD topology failover

 - Timebase (TB) errors

    - TB parity/residue error
    - TFMR parity and firmware control error
    - DEC/HDEC/PURR/SPURR parity errors

HMI handling
============

A core/NX/NPU checkstops are reported as malfunction alert (HMER bit 0).
OPAL handler scans through Fault Isolation Register (FIR) for each
core/nx/npu to detect the exact reason for checkstop and reports it back
to the host alongwith the disposition.

A processor recovery is reported through HMER bits 2, 3 and 11. These are
just an informational messages and no extra recovery is required.

Timer facility errors are reported through HMER bit 4. These are all
recoverable errors. The exact reason for the errors are stored in
Timer Facility Management Register (TFMR). Some of the Timer facility
errors affects TB and some of them affects TOD. TOD is a per chip
Time-Of-Day logic that holds the actual time value of the chip and
communicates with every TOD in the system to achieve synchronized
timer value within a system. TB is per core register (64-bit) derives its
value from ChipTOD at startup and then it gets periodically incremented
by STEP signal provided by the TOD. In a multi-socket system TODs are
always configured as master/backup TOD under primary/secondary
topology configuration respectively.

TB error generates HMI on all threads of the affected core. TB errors
except DEC/HDEC/PURR/SPURR parity errors, causes TB to stop running
making it invalid. As part of TB recovery, OPAL hmi handler synchronizes
with all threads, clears the TB errors and then re-sync the TB with TOD
value putting it back in running state.

TOD errors generates HMI on every core/thread of affected chip. The reason
for TOD errors are stored in TOD ERROR register (0x40030). As part of the
recovery OPAL hmi handler clears the TOD error and then requests new TOD
value from another running chipTOD in the system. Sometimes, if a primary
chipTOD is in error, it may need a TOD topology switch to recover from
error. A TOD topology switch basically makes a backup as new active master.

.. _OPAL_HANDLE_HMI:

OPAL_HANDLE_HMI
===============

.. code-block:: c

   #define OPAL_HANDLE_HMI	98

   int64_t opal_handle_hmi(void);


Superseded by :ref:`OPAL_HANDLE_HMI2`, meaning that :ref:`OPAL_HANDLE_HMI`
should only be called if :ref:`OPAL_HANDLE_HMI2` is not available.

Since :ref:`OPAL_HANDLE_HMI2` has been available since the start of POWER9
systems being supported, if you only target POWER9 and above, you can
assume the presence of :ref:`OPAL_HANDLE_HMI2`.

.. _OPAL_HANDLE_HMI2:

OPAL_HANDLE_HMI2
================

.. code-block:: c

   #define OPAL_HANDLE_HMI2	166

   int64_t opal_handle_hmi2(__be64 *out_flags);

When OS host gets an Hypervisor Maintenance Interrupt (HMI), it must call
:ref:`OPAL_HANDLE_HMI` or :ref:`OPAL_HANDLE_HMI2`. The :ref:`OPAL_HANDLE_HMI`
is an old interface. :ref:`OPAL_HANDLE_HMI2` is newly introduced opal call
that returns direct info to the OS. It returns a 64-bit flag mask currently
set to provide info about which timer facilities were lost, and whether an
event was generated. This information will help OS to take respective
actions.

In case where opal hmi handler is unable to recover from TOD or TB errors,
it would flag :ref:`OPAL_HMI_FLAGS_TOD_TB_FAIL` to indicate OS that TB is
dead. This information then can be used by OS to make sure that the
functions relying on TB value (e.g. udelay()) are aware of TB not ticking.
This will avoid OS getting stuck or hang during its way to panic path.


Parameters
^^^^^^^^^^

.. code-block:: c

   __be64 *out_flags;

Returns the 64-bit flag mask that provides info about which timer facilities
were lost, and whether an event was generated.

.. code-block:: c

   /* OPAL_HANDLE_HMI2 out_flags */
   enum {
        OPAL_HMI_FLAGS_TB_RESYNC        = (1ull << 0), /* Timebase has been resynced */
        OPAL_HMI_FLAGS_DEC_LOST         = (1ull << 1), /* DEC lost, needs to be reprogrammed */
        OPAL_HMI_FLAGS_HDEC_LOST        = (1ull << 2), /* HDEC lost, needs to be reprogrammed */
        OPAL_HMI_FLAGS_TOD_TB_FAIL      = (1ull << 3), /* TOD/TB recovery failed. */
        OPAL_HMI_FLAGS_NEW_EVENT        = (1ull << 63), /* An event has been created */
   };

.. _OPAL_HMI_FLAGS_TOD_TB_FAIL:

OPAL_HMI_FLAGS_TOD_TB_FAIL
  The Time of Day (TOD) / Timebase facility has failed. This is probably fatal
  for the OS, and requires the OS to be very careful to not call any function
  that may rely on it, usually as it heads down a `panic()` code path.
  This code path should be :ref:`OPAL_CEC_REBOOT2` with the OPAL_REBOOT_PLATFORM_ERROR
  option. Details of the failure are likely delivered as part of HMI events if
  `OPAL_HMI_FLAGS_NEW_EVENT` is set.
