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

Zheng, Lv
2015-04-29T00:50:30Z

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.
> > --