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  IRQ-flags state tracing

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  :Author: started by Ingo Molnar <mingo@redhat.com>

  The "irq-flags tracing" feature "traces" hardirq and softirq state, in

  that it gives interested subsystems an opportunity to be notified of
  every hardirqs-off/hardirqs-on, softirqs-off/softirqs-on event that
  happens in the kernel.
  
  CONFIG_TRACE_IRQFLAGS_SUPPORT is needed for CONFIG_PROVE_SPIN_LOCKING
  and CONFIG_PROVE_RW_LOCKING to be offered by the generic lock debugging
  code. Otherwise only CONFIG_PROVE_MUTEX_LOCKING and
  CONFIG_PROVE_RWSEM_LOCKING will be offered on an architecture - these
  are locking APIs that are not used in IRQ context. (the one exception
  for rwsems is worked around)

  Architecture support for this is certainly not in the "trivial"

  category, because lots of lowlevel assembly code deal with irq-flags
  state changes. But an architecture can be irq-flags-tracing enabled in a
  rather straightforward and risk-free manner.
  
  Architectures that want to support this need to do a couple of
  code-organizational changes first:

  - add and enable TRACE_IRQFLAGS_SUPPORT in their arch level Kconfig file
  
  and then a couple of functional changes are needed as well to implement
  irq-flags-tracing support:
  
  - in lowlevel entry code add (build-conditional) calls to the
    trace_hardirqs_off()/trace_hardirqs_on() functions. The lock validator
    closely guards whether the 'real' irq-flags matches the 'virtual'
    irq-flags state, and complains loudly (and turns itself off) if the
    two do not match. Usually most of the time for arch support for
    irq-flags-tracing is spent in this state: look at the lockdep
    complaint, try to figure out the assembly code we did not cover yet,
    fix and repeat. Once the system has booted up and works without a
    lockdep complaint in the irq-flags-tracing functions arch support is
    complete.
  - if the architecture has non-maskable interrupts then those need to be
    excluded from the irq-tracing [and lock validation] mechanism via
    lockdep_off()/lockdep_on().

  In general there is no risk from having an incomplete irq-flags-tracing

  implementation in an architecture: lockdep will detect that and will
  turn itself off. I.e. the lock validator will still be reliable. There
  should be no crashes due to irq-tracing bugs. (except if the assembly
  changes break other code by modifying conditions or registers that

  shouldn't be)