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

Don Zickus
2010-09-14T13:38:21Z

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