Re: [V6][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

Robert Richter
2011-09-28T10:31:50Z

On 23.09.11 15:17:13, Don Zickus wrote:
> @@ -89,6 +89,15 @@ static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs)
>
> handled += a->handler(type, regs);
>
> + /*
> + * Optimization: only loop once if this is not a
> + * back-to-back NMI. The idea is nothing is dropped
> + * on the first NMI, only on the second of a back-to-back
> + * NMI. No need to waste cycles going through all the
> + * handlers.
> + */
> + if (!b2b && handled)
> + break;

I don't think we can leave this in. As said, there are cases that 2
nmis trigger but the handler is called only once. Only the first would
be handled then, and the second get lost cause there is no 2nd nmi
call.

> }
>
> rcu_read_unlock();
> @@ -251,7 +260,13 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
> {
> int handled;
>
> - handled = nmi_handle(NMI_UNKNOWN, regs);
> + /*
> + * Use 'false' as back-to-back NMIs are dealt with one level up.
> + * Of course this makes having multiple 'unknown' handlers useless
> + * as only the first one is ever run (unless it can actually determine
> + * if it caused the NMI)
> + */
> + handled = nmi_handle(NMI_UNKNOWN, regs, false);
> if (handled)
> return;
> #ifdef CONFIG_MCA
> @@ -274,19 +289,49 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
> pr_emerg("Dazed and confused, but trying to continue\n");
> }
>
> +static DEFINE_PER_CPU(bool, swallow_nmi);
> +static DEFINE_PER_CPU(unsigned long, last_nmi_rip);
> +
> static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> {
> unsigned char reason = 0;
> int handled;
> + bool b2b = false;
>
> /*
> * CPU-specific NMI must be processed before non-CPU-specific
> * NMI, otherwise we may lose it, because the CPU-specific
> * NMI can not be detected/processed on other CPUs.
> */
> - handled = nmi_handle(NMI_LOCAL, regs);
> - if (handled)
> +
> + /*
> + * Back-to-back NMIs are interesting because they can either
> + * be two NMI or more than two NMIs (any thing over two is dropped
> + * due to NMI being edge-triggered). If this is the second half
> + * of the back-to-back NMI, assume we dropped things and process
> + * more handlers. Otherwise reset the 'swallow' NMI behaviour
> + */
> + if (regs->ip == __this_cpu_read(last_nmi_rip))
> + b2b = true;
> + else
> + __this_cpu_write(swallow_nmi, false);
> +
> + __this_cpu_write(last_nmi_rip, regs->ip);

Just a minor thing and if you make a new version of this patch: You
could move the write to the else branch.

-Robert

--
Advanced Micro Devices, Inc.
Operating System Research Center