Messages in this thread | | | Date | Wed, 28 Sep 2011 12:31:40 +0200 | From | Robert Richter <> | Subject | Re: [V6][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs |
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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
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