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100 lines
3.8 KiB
ReStructuredText
100 lines
3.8 KiB
ReStructuredText
.. _watchdog-hld:
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Watchdog Virtualization in Device Model
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#######################################
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This document describes the watchdog virtualization implementation in
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ACRN device model.
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Overview
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********
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A watchdog is an important hardware component in embedded systems, used
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to monitor the system's running status, and resets the processor if the
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software crashes. In general, hardware watchdogs rely on a piece of
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software running on the machine which must "kick" the watchdog device
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regularly, say every 10 seconds. If the watchdog doesn't get "kicked"
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after 60 seconds, for example, then the watchdog device asserts the
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RESET line which results in a hard reboot.
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For ACRN we emulate the watchdog hardware in the Intel 6300ESB chipset
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as a PCI device called 6300ESB watchdog and is added into the Device
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Model following the PCI device framework. The following
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:numref:`watchdog-device` shows the watchdog device workflow:
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.. figure:: images/watchdog-image2.png
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:align: center
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:width: 900px
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:name: watchdog-device
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Watchdog device flow
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The DM in the Service VM treats the watchdog as a passive device.
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It receives read/write commands from the watchdog driver, does the
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actions, and returns. In ACRN, the commands are from User VM
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watchdog driver.
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User VM Watchdog Workflow
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*************************
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When the User VM does a read or write operation on the watchdog device's
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registers or memory space (Port IO or Memory map I/O), it will trap into
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the hypervisor. The hypervisor delivers the operation to the Service VM/DM
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through IPI (inter-process interrupt) or shared memory, and the DM
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dispatches the operation to the watchdog emulation code.
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After the DM watchdog finishes emulating the read or write operation, it
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then calls ``ioctl`` to the Service VM/kernel (``/dev/acrn_vhm``). VHM will call a
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hypercall to trap into the hypervisor to tell it the operation is done, and
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the hypervisor will set User VM-related VCPU registers and resume the User VM so the
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User VM watchdog driver will get the return values (or return status). The
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:numref:`watchdog-workflow` below is a typical operation flow:
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from a User VM to the Service VM and return back:
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.. figure:: images/watchdog-image1.png
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:align: center
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:width: 900px
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:name: watchdog-workflow
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Watchdog operation workflow
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Implementation in ACRN and How to Use It
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****************************************
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In ACRN, the Intel 6300ESB watchdog device emulation is added into the
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DM PCI device tree. Its interface structure is (see
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``devicemodel/include/pci_core.h``):
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.. code-block:: c
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struct pci_vdev_ops pci_ops_wdt = {
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.class_name = "wdt-i6300esb",
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.vdev_init = pci_wdt_init,
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.vdev_deinit = pci_wdt_deinit,
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.vdev_cfgwrite = pci_wdt_cfg_write,
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.vdev_cfgread = pci_wdt_cfg_read,
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.vdev_barwrite = pci_wdt_bar_write,
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.vdev_barread = pci_wdt_bar_read
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};
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All functions follow the ``pci_vdev_ops`` definitions for PCI device
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emulation.
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The main part in the watchdog emulation is the timer thread. It emulates
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the watchdog device timeout management. When it gets the kick action
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from the User VM, it resets the timer. If the timer expires before getting a
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timely kick action, it will call DM API to reboot that User VM.
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In the User VM launch script, add: ``-s xx,wdt-i6300esb`` into DM parameters.
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(xx is the virtual PCI BDF number as with other PCI devices)
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Make sure the User VM kernel has the I6300ESB driver enabled:
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``CONFIG_I6300ESB_WDT=y``. After the User VM boots up, the watchdog device
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will be created as node ``/dev/watchdog``, and can be used as a normal
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device file.
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Usually the User VM needs a watchdog service (daemon) to run in userland and
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kick the watchdog periodically. If something prevents the daemon from
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kicking the watchdog, for example the User VM system is hung, the watchdog
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will timeout and the DM will reboot the User VM.
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