Because ACRN is a flexible, lightweight reference hypervisor, we provide limited timer management services:

  • Only lapic tsc-deadline timer is supported as the clock source.
  • A timer can only be added on the logical CPU for a process or thread. Timer scheduling or timer migrating are not supported.

How it works

When the system boots, we check that the hardware supports lapic tsc-deadline timer by checking CPUID.01H:ECX.TSC_Deadline[bit 24]. If support is missing, we output an error message and panic the hypervisor. If supported, we register the timer interrupt callback that raises a timer softirq on each logical CPU and set the lapic timer mode to tsc-deadline timer mode by writing the local APIC LVT register.

Data Structures and APIs


API link to hv_timer and per_cpu_timer structs in include/arch/x86/timer.h And to the function APIs there too.

Before adding a timer, we must initialize the timer with initialize_timer. The processor generates a timer interrupt when the value of timer-stamp counter is greater than or equal to the fire_tsc field. If you want to add a periodic timer, you should also pass the period (unit in tsc cycles), otherwise, period_in_cycle will be ignored. When the timer interrupt is generated, it will call the callback function func with parameter priv_data.

The initialize_timer function only initialize the timer data structure; it will not program the IA32_TSC_DEADLINE_MSR to generate the timer interrupt. If you want to generate a timer interrupt, you must call add_timer to add the timer to the per_cpu_timer timer_list. In return, we will chose the nearest expired timer on the timer_list and program IA32_TSC_DEADLINE_MSR by writing its value to fire_ts. Then when the fire_tsc expires, it raises the interrupt whose callback raises a softirq. We will handle the software interrupt before the VM reenters the guest. (Currently, the hypervisor only uses the timer for the console).

The timer softirq handler will check each expired timer on its timer_list. Before calling the expired timer callback handler, it will remove the timer from its logical cpu timer_list. After calling the timer callback handler, it will re-add the timer to the timer_list if it’s a periodic timer. If you want to modify a timer before it expires, you should call del_timer to remove the timer from the timer_list, then call add_timer again after updating the timer fields.


Only call initialize_timer only once for each timer. Don’t call add_timer or del_timer in the timer callback function.