linux-kernel - Re: [PATCH] x86, nmi: workaround sti; hlt race vs nmi; intr

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Date:	Mon, 27 Sep 2010 16:17:15 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Joerg Roedel <joro@...tes.org>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] x86, nmi: workaround sti; hlt race vs nmi; intr

  On 09/27/2010 12:31 PM, Joerg Roedel wrote:
> On Sun, Sep 19, 2010 at 06:28:19PM +0200, Avi Kivity wrote:
> >  On machines without monitor/mwait we use an sti; hlt sequence to atomically
> >  enable interrupts and put the cpu to sleep.  The sequence uses the "interrupt
> >  shadow" property of the sti instruction: interrupts are enabled only after
> >  the instruction following sti has been executed.  This means an interrupt
> >  cannot happen in the middle of the sequence, which would leave us with
> >  the interrupt processed but the cpu halted.
> >
> >  The interrupt shadow, however, can be broken by an nmi; the following
> >  sequence
> >
> >     sti
> >       nmi ... iret
> >       # interrupt shadow disabled
> >       intr ... iret
> >     hlt
> >
> >  puts the cpu to sleep, even though the interrupt may need additional
> >  processing after the hlt (like scheduling a task).
>
> Doesn't the interrupt return path check for a re-schedule condition
> before iret? So to my believe the handler would not jump back to the
> idle task if something else becomes running in the interrupt handler,
> no?
>

Perhaps on preemptible kernels?  But at least on non-preemptible 
kernels, you can't just switch tasks while running kernel code.

void cpu_idle(void)
{
     current_thread_info()->status |= TS_POLLING;

     /*
      * If we're the non-boot CPU, nothing set the stack canary up
      * for us.  CPU0 already has it initialized but no harm in
      * doing it again.  This is a good place for updating it, as
      * we wont ever return from this function (so the invalid
      * canaries already on the stack wont ever trigger).
      */
     boot_init_stack_canary();

     /* endless idle loop with no priority at all */
     while (1) {
         tick_nohz_stop_sched_tick(1);
         while (!need_resched()) {

             rmb();

             if (cpu_is_offline(smp_processor_id()))
                 play_dead();
             /*
              * Idle routines should keep interrupts disabled
              * from here on, until they go to idle.
              * Otherwise, idle callbacks can misfire.
              */
             local_irq_disable();
             enter_idle();
             /* Don't trace irqs off for idle */
             stop_critical_timings();
             pm_idle();
             start_critical_timings();

             trace_power_end(smp_processor_id());

             /* In many cases the interrupt that ended idle
                has already called exit_idle. But some idle
                loops can be woken up without interrupt. */
             __exit_idle();
         }

         tick_nohz_restart_sched_tick();
         preempt_enable_no_resched();
         schedule();
         preempt_disable();
     }
}

Looks like we rely on an explicit schedule() - pm_idle() is called with 
preemption disabled.

(pm_idle eventually calls safe_halt() if no other idle method is used)

-- 
error compiling committee.c: too many arguments to function

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