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

Zheng, Lv
2015-04-28T00:44:18Z

Hi,

> From: Borislav Petkov [mailto:bp@alien8.de]
> 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.

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.
> --