doc: update HLD Timer section

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>

doc/.known-issues/doc/dupdecl.conf

@@ -2,4 +2,4 @@
 # Emulated devices
 #
 #
-^(?P<filename>[-._/\w]+/hld/hld-io-emulation.rst):(?P<lineno>[0-9]+): WARNING: Duplicate declaration.
+^(?P<filename>[-._/\w]+/hld/hv-io-emulation.rst):(?P<lineno>[0-9]+): WARNING: Duplicate declaration.

doc/developer-guides/hld/hld-hypervisor.rst

@@ -9,6 +9,7 @@ Hypervisor high-level design
 
    hv-startup
    hv-cpu-virt
-   Memory management <memmgt-hld>
-   I/O Emulation <hld-io-emulation>
-   Physical Interrupt <interrupt-hld>
+   Memory management <hv-memmgt>
+   I/O Emulation <hv-io-emulation>
+   Physical Interrupt <hv-interrupt>
+   Timer <hv-timer>

doc/developer-guides/hld/hv-timer.rst

@@ -0,0 +1,60 @@
+.. _timer-hld:
+
+Timer
+#####
+
+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
+************************
+
+.. note:: 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.
+
+.. note::
+
+   Only call initialize_timer only once for each timer.
+   Don't call add_timer or del_timer in the timer callback function.