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

Mikulas Patocka
2007-10-16T10:34:13Z

On Tue, 16 Oct 2007, Nick Piggin wrote:

> > > 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?
>
> Also, for non-wb memory. I don't think the Intel document referenced
> says anything about this, but the AMD document says that loads can pass
> loads (page 8, rule b).
>
> This is why our rmb() is still an lfence.

I see, AMD says that WC memory loads can be out-of-order.

There is very little usability to it --- framebuffer and AGP aperture is
the only piece of memory that is WC and no kernel structures are placed
there, so it is possible to remove that lfence.

Mikulas