RE: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader

[Zheng, Lv]

Hi,

> From: Zheng, Lv
> Sent: Tuesday, April 28, 2015 8:44 AM
> 
> Hi,
> 
> > From: Borislav Petkov [mailto:bp@xxxxxxxxx]
> > Sent: Monday, April 27, 2015 4:47 PM
> >
> > On Mon, Apr 27, 2015 at 03:16:00AM +0000, Zheng, Lv wrote:
> > > > @@ -840,7 +840,9 @@ static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
> > > >  	struct ghes *ghes;
> > > >  	int sev, ret = NMI_DONE;
> > > >
> > > > -	raw_spin_lock(&ghes_nmi_lock);
> > > > +	if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
> > > > +		return ret;
> > > > +
> > >
> > > Just a simple question.
> > > Why not just using cmpxchg here instead of atomic_add_unless so that no atomic_dec will be needed.
> >
> > What do you think atomic_add_unless ends up doing:
> >
> > #APP
> > # 177 "./arch/x86/include/asm/atomic.h" 1
> > 	.pushsection .smp_locks,"a"
> > .balign 4
> > .long 671f - .
> > .popsection
> > 671:
> > 	lock; cmpxchgl %edx,ghes_in_nmi(%rip)	# D.37056, MEM[(volatile u32 *)&ghes_in_nmi]
> > # 0 "" 2
> > #NO_APP
> >
> > And you need to atomic_dec() so that another reader can enter, i.e. how
> > the exclusion primitive works.
> >
> > Or did you have something else in mind?
> 
> My mistake.
> I mean cmpxchg() and xchg() (or atomic_cmpxchg() and atomic_xchg()) pair here, so nothing can be reduced.

Let me correct, it should be atomic_cmpxchg() and atomic_set() here as you only need to switch between 0 and 1.
Sorry for the noise.

Thanks and best regards
-Lv

> 
> But IMO, atomic_add_unless() is implemented via cmpxchg on many architectures.
> And it might be better to use it directly here which is a bit faster as you actually only need one value switch here.
> 
> Thanks and best regards
> -Lv
> 
> 
> >
> > --
> > Regards/Gruss,
> >     Boris.
> >
> > ECO tip #101: Trim your mails when you reply.
> > --
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