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

Peter Zijlstra
2011-08-22T14:23:09Z

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.