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

Don Zickus
2011-09-28T12:37:58Z

On Wed, Sep 28, 2011 at 12:31:40PM +0200, Robert Richter wrote:
> 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.

Right. Avi, Jeremy what was your objection that needed this optimization
in the first place?

>
> > }
> >
> > 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.

Ah, true. Thanks.

Cheers,
Don