The monitor component is a drop-in replacement for init that allows for the
inspection and debugging of child components. Its configuration mirrors the
configuration of init.

In contrast to init, the monitor requests the 'platform_info' ROM to detect
the used kernel at runtime. This is needed because low-level mechanisms for
interacting with threads differ between the various kernels supported by
Genode. Note that Linux is not supported by the monitor.

A component can be selected for monitoring by routing its PD and CPU sessions
to the monitor's local PD and CPU services instead of the routing those
sessions to the parent. For example, the following start node selects the
'test-log' component to be monitored.

! <start name="first-test-log">
!   <resource name="RAM" quantum="2M"/>
!   <binary name="test-log"/>
!   <route>
!     <service name="PD">  <local/> </service>
!     <service name="CPU"> <local/> </service>
!     <any-service> <parent/> </any-service>
!   </route>
! </start>

In case a monitored component uses VM sessions (e.g. VirtualBox), the VM
service has to be routed to '<local/>' too.

If monitor's configuration features a <monitor> sub node, the monitor creates
a terminal session that offers access to the monitored components via the GDB
protocol. Each component appears as a distinct GDB inferior. The following
GDB commands are useful:

* List all monitored components:
  ! (gdb) info inferiors

* List all threads:
  ! (gdb) info threads

* Select a component for inspection, specifying the component's inferior ID:
  ! (gdb) inferior 2

* Show 10 words of the component's memory at virtual address 0x100000:
  ! x/10x 0x100000

The <monitor> configuration can host optional <policy> nodes referring to
inferiors by their respective labels. For example:

! <monitor max_response="2K">
!  <policy label="first-test-log" wait="no" stop="yes" wx="no"/>
! </monitor>

By setting the 'wait' attribute to "yes", the execution of the inferior is
delayed until the monitor receives the GDB command for continuing execution.
This is useful for inspecting the startup phase of a component. By default,
inferiors don't wait. If this attribute is set, the 'wx' attribute needs to
be set as well because a software breakpoint is used to stop the inferior
at the first instruction.

;;; XXX not implemented, uncomment once completed
;
; The 'stop' attribute defines the behavior of an inferior when GDB connects
; to the monitor. By default, all inferiors are stopped. By setting the
; attribute to "no", a GDB connection does not interfere with the execution of
; the inferiors while memory can be inspected.

The enabling of the 'wx' attribute prompts the monitor to map the executable
code of the monitored component as writeable memory, allowing the patching of
text segment by GDB, which is needed for using breakpoints.

The "max_response" attribute of the <monitor> node specifies the
maximum payload size of a GDB command response packet. The default value
is 2048 bytes. It can be increased for higher memory dump throughput, provided
that the terminal session component has the capacity to receive the configured
amount plus GDB protocol overhead and potential asynchronous notifications.

RAM wiping
----------

As a secondary functionality, the monitor wipes all RAM dataspaces allocated
by the monitored components when the RAM is freed. This is useful as a possible
defense against cold-boot attacks. Note however that only RAM allocated by
monitored components is subjected to the wiping. Shared-memory buffers
obtained from a service - think of a GUI server - are freed by the service,
not the client. If the service is not monitored, client information could
prevail in such a shared-memory buffer.
