Hi,

> From: Zheng, Lv
> Sent: Wednesday, April 29, 2015 8:25 AM
> Subject: RE: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader
> 
> Hi,
> 
> > From: Borislav Petkov [mailto:bp@alien8.de]
> > Sent: Tuesday, April 28, 2015 9:59 PM
> > Subject: Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader
> >
> > On Tue, Apr 28, 2015 at 01:38:41PM +0000, Zheng, Lv wrote:
> > > > -	raw_spin_lock(&ghes_nmi_lock);
> > > > +	if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
> > > > +		return ret;
> > > > +
> > >
> > > if (atomic_cmpxchg(&ghes_in_nmi, 0, 1))
> > > 	return ret;
> >
> > Ok, now I understand what you mean.
> >
> > We absolutely want to use atomic_add_unless() because we get to save us
> > the expensive
> >
> > 	LOCK; CMPXCHG
> >
> > if the value was already 1. Which is exactly what this patch is trying
> > to avoid - a thundering herd of cores CMPXCHGing a global variable.
> 
> IMO, on most architectures, the "cmp" part should work just like what you've done with "if".
> And on some architectures, if the "xchg" doesn't happen, the "cmp" part even won't cause a pipe line hazard.

If you man the LOCK prefix, I understand now.

Thanks and best regards
-Lv

> 
> Thanks and best regards
> -Lv
> 
> 
> >
> > I.e.,
> >
> > 	movl	ghes_in_nmi(%rip), %ecx	# MEM[(const int *)&ghes_in_nmi], c
> > 	cmpl	$1, %ecx	#, c
> > 	je	.L311	#,				<--- exit here if ghes_in_nmi == 1.
> > 	leal	1(%rcx), %edx	#, D.37163
> > 	movl	%ecx, %eax	# c, c
> > #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.37163, MEM[(volatile u32 *)&ghes_in_nmi]
> > # 0 "" 2
> > #NO_APP
> >
> > --
> > Regards/Gruss,
> >     Boris.
> >
> > ECO tip #101: Trim your mails when you reply.
> > --
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