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transcode, edit, and upload HLD 0.7 section 3.6 (Timer) Also, fix the hv sections file names to be consistent. Tracked-on: #1623 Signed-off-by: David B. Kinder <david.b.kinder@intel.com>
61 lines
2.6 KiB
ReStructuredText
61 lines
2.6 KiB
ReStructuredText
.. _timer-hld:
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Timer
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#####
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Because ACRN is a flexible, lightweight reference hypervisor, we provide
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limited timer management services:
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- Only lapic tsc-deadline timer is supported as the clock source.
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- A timer can only be added on the logical CPU for a process or thread. Timer
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scheduling or timer migrating are not supported.
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How it works
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************
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When the system boots, we check that the hardware supports lapic
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tsc-deadline timer by checking CPUID.01H:ECX.TSC_Deadline[bit 24]. If
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support is missing, we output an error message and panic the hypervisor.
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If supported, we register the timer interrupt callback that raises a
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timer softirq on each logical CPU and set the lapic timer mode to
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tsc-deadline timer mode by writing the local APIC LVT register.
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Data Structures and APIs
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************************
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.. note:: API link to hv_timer and per_cpu_timer structs in include/arch/x86/timer.h
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And to the function APIs there too.
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Before adding a timer, we must initialize the timer with
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*initialize_timer*. The processor generates a timer interrupt when the
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value of timer-stamp counter is greater than or equal to the *fire_tsc*
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field. If you want to add a periodic timer, you should also pass the
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period (unit in tsc cycles), otherwise, period_in_cycle will be ignored.
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When the timer interrupt is generated, it will call the callback
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function *func* with parameter *priv_data*.
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The *initialize_timer* function only initialize the timer data
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structure; it will not program the ``IA32_TSC_DEADLINE_MSR`` to generate
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the timer interrupt. If you want to generate a timer interrupt, you must
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call *add_timer* to add the timer to the *per_cpu_timer* timer_list. In
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return, we will chose the nearest expired timer on the timer_list and
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program ``IA32_TSC_DEADLINE_MSR`` by writing its value to fire_ts. Then
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when the fire_tsc expires, it raises the interrupt whose callback raises
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a softirq. We will handle the software interrupt before the VM reenters
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the guest. (Currently, the hypervisor only uses the timer for the
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console).
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The timer softirq handler will check each expired timer on its
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timer_list. Before calling the expired timer callback handler, it will
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remove the timer from its logical cpu timer_list. After calling the
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timer callback handler, it will re-add the timer to the timer_list if
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it’s a periodic timer. If you want to modify a timer before it expires,
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you should call del_timer to remove the timer from the timer_list, then
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call add_timer again after updating the timer fields.
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.. note::
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Only call initialize_timer only once for each timer.
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Don't call add_timer or del_timer in the timer callback function.
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