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 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.


#define OPAL_HANDLE_HMI      98

int64_t opal_handle_hmi(void);

Superseded by OPAL_HANDLE_HMI2, meaning that OPAL_HANDLE_HMI should only be called if OPAL_HANDLE_HMI2 is not available.

Since 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 OPAL_HANDLE_HMI2.


#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 OPAL_HANDLE_HMI or OPAL_HANDLE_HMI2. The OPAL_HANDLE_HMI is an old interface. 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 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.


__be64 *out_flags;

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

/* 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 */

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 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.