Re: [RFC][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

Don Zickus
2011-08-22T15:25:34Z

On Mon, Aug 22, 2011 at 04:22:15PM +0200, Peter Zijlstra wrote:
> On Fri, 2011-08-19 at 16:37 -0400, Don Zickus wrote:
> > @@ -260,6 +260,8 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
> > pr_emerg("Dazed and confused, but trying to continue\n");
> > }
> >
> > +DEFINE_PER_CPU(bool, swallow_nmi);
> > +
> > static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> > {
> > unsigned char reason = 0;
> > @@ -271,8 +273,28 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> > * NMI can not be detected/processed on other CPUs.
> > */
> > handled = nmi_handle(NMI_LOCAL, regs);
> > - if (handled)
> > + if (handled) {
> > + /*
> > + * When handling multiple NMI events, we are not
> > + * sure if the second NMI was dropped (because of
> > + * too many NMIs), piggy-backed on the same NMI
> > + * (perf) or is queued right behind this NMI.
> > + * In the last case, we may accidentally get an
> > + * unknown NMI because the event is already handled.
> > + * Flag for this condition and swallow it later.
> > + *
> > + * FIXME: This detection has holes in it mainly
> > + * because we can't tell _when_ the next NMI comes
> > + * in. A multi-handled NMI event followed by an
> > + * unknown NMI a second later, clearly should not
> > + * be swallowed.
> > + */
> > + if (handled > 1)
> > + __this_cpu_write(swallow_nmi, true);
> > + else
> > + __this_cpu_write(swallow_nmi, false);
> > return;
> > + }
> >
> > /* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
> > raw_spin_lock(&nmi_reason_lock);
> > @@ -296,6 +318,8 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> > raw_spin_unlock(&nmi_reason_lock);
> >
> > unknown_nmi_error(reason, regs);
> > +
> > + __this_cpu_write(swallow_nmi, false);
> > }
>
> All writes, no reads... the actual dropping of NMIs got lost and now
> lives in patch 5 which purports to be about statistics only.

Oops. I screwed up when breaking up the changes into multiple patches.
I'll fix that. Thanks for catching that.

Cheers,
Don