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

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Date:	Mon, 13 Sep 2010 10:09:30 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Don Zickus <dzickus@...hat.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 Sat, 2010-09-11 at 02:29 +0800, Don Zickus wrote:
> On Fri, Sep 10, 2010 at 06:03:56PM +0200, Andi Kleen wrote:
> > On Fri, 10 Sep 2010 11:56:05 -0400
> > Don Zickus <dzickus@...hat.com> wrote:
> > 
> > > On Fri, Sep 10, 2010 at 10:51:03AM +0800, Huang Ying wrote:
> > > > The original NMI handler is quite outdated in many aspects. This
> > > > patch try to fix it.
> > > > 
> > > > In original code, NMI reason io port (0x61) is only processed on
> > > > BSP. This makes it impossible to hot-remove BSP. To solve the issue,
> > > > a raw spinlock is used to make the port can be processed on any CPU.
> > > 
> > > Do we really want to use a spinlock inside the nmi handler?
> > 
> > As long as it's only between CPUs
> > (that is only ever used between different NMI handlers) 
> > that's fine.  It's certainly safer than having races between CPUs.
> > 
> > > I thought the NMIs sent to the io port are only routed to one cpu as
> > > determined by the io-apic?  Is it spread out to other cpus now?
> > 
> > There can be cases where it can happen I believe.
> 
> 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?

Best Regards,
Huang Ying 

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