[RFC] Latched NMI handler

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

Hi!

Given the recent interest for latched NMI handler providing the ability
to fault and take exception within NMI handlers, I thought it would be
good if I wrote down the pseudo-code I got stucked in my brain since
this last discussion with Linus on the topic about a year ago. Feedback
is welcome, and hopefully it will be useful to Steven who is starting to
work a solution.

variables used:

stack-local int nmi_nest_count;
stack-local int nmi_latch;
__nmi_epilogue_begin (pointer to text)
__nmi_epilogue_end (pointer to text)
REAL_NMI_STACK: beginning of the stack used for real nmi handler
LATCHED_NMI_STACK: beginning of the stack used for latched nmi handler

int in_nmi_epilogue(void)
{
  return (instruction_pointer() >= __nmi_epilogue_begin
		&& instruction_pointer() < __nmi_epilogue_end);
}

int in_nmi(void)
{
  return nmi_nest_count > 0;
}

/* Use REAL_NMI_STACK */
real_nmi_handler: /* always running with nmis disabled */
  /*
   * We disable interrupts to ensure we don't have to deal with IRQs
   * when NMIs get re-enabled due to an iret from a fault/exception.
   */
  local_irq_disable();
  if (in_nmi_epilogue()) {
    nmi_latch = 0;
    /* set stack pointer to start of LATCHED_NMI_STACK */
    goto latched_nmi_handler;
  }
  if (in_nmi()) {
     nmi_latch = 1;
     iret
  }
  nmi_nest_count++;
  /* set stack pointer to start of LATCHED_NMI_STACK */
  goto latched_nmi_handler;


/* Use LATCHED_NMI_STACK */
latched_nmi_handler:	/* Can fault and reenable NMIs. */

  [ execute actual system NMI handler, including faults, int3, ... ]

  /*
   * note: test nmi_latch and iret instruction are within the
   * epilogue range to deal with latch test vs iret non-atomicity. If a
   * real nmi nests over this range, it clears the nmi_latch flag and
   * just restarts the latched nmi handler.  No
   * faults/exceptions/interrupts are permitted in this region, except
   * for the real NMI and MCEs (TODO).
   */
__nmi_epilogue_begin:
  /*
   * here we are restarting the latched nmi handler if an nmi happened
   * while nested within the nmi nest count.
   */
  if (nmi_latch) {
    nmi_latch = 0;
    goto latched_nmi_handler;
  }
  nmi_nest_count--;
  iret  /* restores interrupts */
__nmi_epilogue_end:

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com