Re: [V5][PATCH 4/6] x86, nmi: add in logic to handle multipleevents and unknown NMIs

[Don Zickus]

On Wed, Sep 21, 2011 at 05:18:30PM +0200, Robert Richter wrote:
> On 21.09.11 10:04:32, Don Zickus wrote:
> > On Wed, Sep 21, 2011 at 12:08:42PM +0200, Robert Richter wrote:
> > > On 20.09.11 10:43:10, Don Zickus wrote:
> > > > @@ -87,6 +87,16 @@ 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;
> > > 
> > > In rare cases we will lose nmis here.
> > > 
> > > We see a back-to-back nmi in the case if a 2 nmi source triggers
> > > *after* the nmi handler is entered. Depending on the internal cpu
> > > timing influenced by microcode and SMM code execution, the nmi may not
> > > entered immediately. So all sources that trigger *before* the nmi
> > > handler is entered raise only one nmi with no subsequent nmi.
> > 
> > Right, but that can only happen with the second NMI in the back-to-back
> > NMI case.  Here the optimization is only for the first NMI, with the
> > assumption that you will always have a second NMI if multiple sources
> > trigger, so you can process those in the second iteration (assuming we
> > correctly detect the back-to-back NMI condition).  Then when the second
> > NMI comes in, we have no idea how many we dropped to get here so we
> > process all the handlers based on the assumption we might not have another
> > NMI behind us to make up for the dropped NMIs.
> > 
> > Unless I misunderstood your point above?
> 
> No, my point was that a second NMI might not be latched even if there
> are two nmi sources pending.
> 
> Your logic is correct but assumes you will always receive a second
> nmi. This is not always the case depending on the cpu's internal
> timing. Usually there is the following sequence for back-to-back nmis:
> 
> 1. HW triggers first NMI, an NMI is pending.
> 2. NMI handler is called, no NMI pending anymore.
> 3. HW triggers a second NMI, an NMI is pending.
> 4. Return from NMI handler.
> 5. NMI handler is called again to serve the 2nd, no NMI pending anymore.
> 6. Return from NMI handler.
> 
> The above is what your algorithm covers.
> 
> But in rare cases there is the following:
> 
> 1. The cpu executes some microcode or SMM code.
> 2. HW triggers the first NMI, an NMI is pending.
> 3. HW triggers a second NMI, the NMI is still pending.
> 4. The cpu finished microcode or SMM code.
> 5. NMI handler is called, no NMI pending anymore.
> 6. Return from NMI handler.
> 
> In this case the handler is called only once and the second nmi
> remains unhandled with you implementation.
> 
> I don't see a way how this could be catched without serving all
> handlers the first time. But as said, in favor of the optimization I
> think we can live with losing some NMIs.

Ah, I get it know.  Crap.  Well I think Avi was pushing it to make those
ticket_spin_locks work in virt land.  It seems like we should lean towards
removing the optimization.  Avi?

Cheers,
Don
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