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

Avi Kivity
2011-10-02T10:08:06Z

On 09/28/2011 03:37 PM, Don Zickus wrote:
> 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?
>

First, iterating over all NMI sources is going to be slow, and a lot
more so in a guest. This reduces the performance of perf.

Second, I wanted to use NMIs as a way of waking up a vcpu sleeping with
interrupts disabled (in the context of Jeremy's paravirt spinlock
patches). Looks like we'll have to use paravirtualization for that.

--
error compiling committee.c: too many arguments to function