[PATCH] i386 linux: make 32-bit PAE kernel work when built with newer gcc

["Jan Beulich" <JBeulich@novell.com>]

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The compiler isn't required to order the two stores ptep_get_and_clear_full() in any particular way, and we saw cases
where the upper 32 bits get stored before the lower ones, which causes the access to fail (page-fault propagated out of
Xen).

Jan

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Index: head-2006-03-13/include/asm-i386/mach-xen/asm/pgtable.h
===================================================================
--- head-2006-03-13.orig/include/asm-i386/mach-xen/asm/pgtable.h	2006-03-13 10:22:14.000000000 +0100
+++ head-2006-03-13/include/asm-i386/mach-xen/asm/pgtable.h	2006-03-13 15:25:53.000000000 +0100
@@ -272,7 +272,15 @@ 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, but the hypervisor
+		   must not see the high part before the low one. */
+		ptep->pte_low = 0;
+		ptep->pte_high = 0;
+#else
 		*ptep = __pte(0);
+#endif
 	} else {
 		pte = ptep_get_and_clear(mm, addr, ptep);
 	}

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