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

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Message-ID: <Pine.LNX.4.64.0710181904540.29362@artax.karlin.mff.cuni.cz>
Date:	Thu, 18 Oct 2007 19:06:09 +0200 (CEST)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Nick Piggin <npiggin@...e.de>
cc:	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: LFENCE instruction (was: [rfc][patch 3/3] x86: optimise barriers)

> > > > You already must not place any data structures into WC memory --- for 
> > > > example, spinlocks wouldn't work there.
> > > 
> > > What do you mean "already"?
> > 
> > I mean "in current kernel" (I checked it in 2.6.22)
> 
> Ahh, that's not "current kernel", though ;)
> 
> 4071c718555d955a35e9651f77086096ad87d498
>
> > So drivers can't assume that wmb() works on write-combining memory.
> 
> Drivers should be able to assume that wmb() orders _everything_ (except
> some whacky Altix thing, which I really want to fold under wmb at some
> point anyway).
> 
> So I decided that old x86 semantics isn't right, and now it really is a
> lock op / sfence everywhere.

I see. I'm just curious --- is there any real usage for WC memory, except 
graphics card memory?

Mikulas
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