linux-kernel - Re: [RFC 4/6] x86, NMI, Rewrite NMI handler

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Date:	Tue, 14 Sep 2010 09:37:56 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 4/6] x86, NMI, Rewrite NMI handler

On Tue, Sep 14, 2010 at 01:12:41PM +0800, Huang Ying wrote:
> On Mon, 2010-09-13 at 22:04 +0800, Don Zickus wrote:
> > On Mon, Sep 13, 2010 at 10:09:30AM +0800, Huang Ying wrote:
> > > > The reason I asked was, I thought it would be easier to have a global
> > > > variable that tells the nmi handler which cpu has the NMI's routed to its
> > > > io port.  This way if you want to swap out the bsp cpu, you could perhaps
> > > > just re-route the nmi to a new cpu and the global variable would be
> > > > updated accordingly?
> > > 
> > > Then we need some kind of protection or race condition between
> > > re-routing NMI and updating the variable. Do you think so?
> > 
> > Well, I thought the only reasonable place to update the variable is when
> > the cpu is being taken offline, during the MTRR update.  Since no NMIs can
> > be processed when the cpu's are syncing their MTRR, there shouldn't be a
> > race condition, no?
> > 
> > Then again I am probably missing something obvious.  Like I don't know how
> > cpu's deal with interrupts/NMIs when they are going offline.
> > 
> > It was just a thought to avoid the spinlock.
> 
> Why do you hate spinlock inside NMI handler? I think it is safe and
> simple if only used in NMI handler.

I guess I always had the mentality that spinlocks in an NMI context was a
big no-no.  Never really thought about if there were safe use-cases or
not.

Cheers,
Don
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