Re: [PATCH] i386: PAE entries must have their low word cleared first

[Hugh Dickins <hugh@veritas.com>]

On Wed, 26 Apr 2006, Jan Beulich wrote:

> During Xen development we noticed that writes to 64-bit variables may get
> coded as high-word-before-low-word writes by gcc. Consequently, when
> invalidating a PAE page table entry, the low word must be explicitly written
> first, as otherwise, as pointed out by Keir, speculative execution may result
> in a partially modified (and still valid) translation to be used, which has,
> according to the specification, the potential of dead-locking the machine.

If that's so (I don't trust my judgement on matters of speculative
execution), then I think you'd do better to replace the *ptep = __pte(0)
by pte_clear(mm, addr, ptep), and so avoid your ugly #ifdef'ing: please
check, but I think you'll find that reduces to just the barrier you want.
CC'ed Zach since it's his optimization, and he'll judge that spexecution.

Hugh

> Signed-off-by: Jan Beulich <jbeulich@novell.com>
> Cc: Keir Fraser <Keir.Fraser@cl.cam.ac.uk>
> 
> diff -Npru /home/jbeulich/tmp/linux-2.6.17-rc2/include/asm-i386/pgtable.h
> 2.6.17-rc2-i386-pae-ptep_get_and_clear_full/include/asm-i386/pgtable.h
> --- /home/jbeulich/tmp/linux-2.6.17-rc2/include/asm-i386/pgtable.h	2006-04-26 10:56:03.000000000 +0200
> +++ 2.6.17-rc2-i386-pae-ptep_get_and_clear_full/include/asm-i386/pgtable.h	2006-04-24 12:28:37.000000000 +0200
> @@ -268,7 +268,15 @@ static inline pte_t ptep_get_and_clear_f
>  	pte_t pte;
>  	if (full) {
>  		pte = *ptep;
> +#ifdef CONFIG_X86_PAE
> +		/* Cannot do this in a single step, as the compiler
> +		   may issue the two stores in either order. */
> +		ptep->pte_low = 0;
> +		barrier();
> +		ptep->pte_high = 0;
> +#else
>  		*ptep = __pte(0);
> +#endif
>  	} else {
>  		pte = ptep_get_and_clear(mm, addr, ptep);
>  	}
> 
> 
> -
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