linux-kernel - Re: [RFC] page-table walkers vs memory order

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Message-ID: <20120730202857.GL2391@linux.vnet.ibm.com>
Date:	Mon, 30 Jul 2012 13:28:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC] page-table walkers vs memory order

On Mon, Jul 30, 2012 at 08:21:40PM +0100, Jamie Lokier wrote:
> Paul E. McKenney wrote:
> > > Does some version of gcc, under the options which we insist upon,
> > > make such optimizations on any of the architectures which we support?
> > 
> > Pretty much any production-quality compiler will do double-fetch
> > and old-value-reuse optimizations, the former especially on 32-bit
> > x86.  I don't know of any production-quality compilers that do value
> > speculation, which would make the compiler act like DEC Alpha hardware,
> > and I would hope that if this does appear, (1) we would have warning
> > and (2) it could be turned off.  But there has been a lot of work on
> > this topic, so we would be foolish to rule it out.
> 
> GCC documentation for IA-64:
> 
>    -msched-ar-data-spec
>    -mno-sched-ar-data-spec
>      (En/Dis)able data speculative scheduling after reload. This results
>      in generation of ld.a instructions and the corresponding check
>      instructions (ld.c / chk.a). The default is 'enable'.
> 
> I don't know if that results in value speculation of the relevant kind.

If I remember correctly, the chk.a instruction will detect failed
speculation via cache state and deal with the situation correctly,
but I really need to defer to someone with more recent IA-64 experience.

							Thanx, Paul

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