Re: LFENCE instruction (was: [rfc][patch 3/3] x86: optimise barriers)

Mikulas Patocka
2007-10-15T22:08:22Z

> On Mon, 15 Oct 2007 22:47:42 +0200 (CEST)
> Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz> wrote:
>
> > > According to latest memory ordering specification documents from
> > > Intel and AMD, both manufacturers are committed to in-order loads
> > > from cacheable memory for the x86 architecture. Hence, smp_rmb()
> > > may be a simple barrier.
> > >
> > > http://developer.intel.com/products/processor/manuals/318147.pdf
> > > http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/24593.pdf
> >
> > Hi
> >
> > I'm just wondering about one thing --- what is LFENCE instruction
> > good for?
> >
> > SFENCE is for enforcing ordering in write-combining buffers (it
> > doesn't have sense in write-back cache mode).
> > MFENCE is for preventing of moving stores past loads.
> >
> > But what is LFENCE for? I read the above documents and they already
> > say that CPUs have ordered loads.
> >
>
> The cpus also have an explicit set of instructions that deliberately do
> unordered stores/loads, and s/lfence etc are mostly designed for those.

I know about unordered stores (movnti & similar) --- they basically use
write-combining method on memory that is normally write-back --- and they
need sfence. But which one instruction does unordered load and needs
lefence?

Mikulas