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

[Mikulas Patocka]

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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/