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

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Date:	Wed, 28 Sep 2011 08:37:20 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Robert Richter <robert.richter@....com>
Cc:	"x86@...nel.org" <x86@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"ying.huang@...el.com" <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"avi@...hat.com" <avi@...hat.com>,
	"jeremy@...p.org" <jeremy@...p.org>
Subject: Re: [V6][PATCH 4/6] x86, nmi:  add in logic to handle multiple
 events and unknown NMIs

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
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