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

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

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.

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);
 	}