x86: pud_clear: only reload cr3 if necessary

Rather than unconditionally reloading cr3, only do so if the pud we're
updating is within the active pgd.

This eliminates TLB flushes most of the time.  The
performance-critical uses of pud_clear are during execve and exit, but
in those cases cr3 is referring to some other pagetable.  The only
other use of pud_clear is during a large (1Gbyte+) munmap, and those
are sufficiently rare that a couple of cr3 reloads won't hurt.
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/asm-x86/pgtable-3level.h

@@ -93,17 +93,20 @@ static inline void native_pmd_clear(pmd_t *pmd)
 
 static inline void pud_clear(pud_t *pudp)
 {
+	unsigned long pgd;
+
 	set_pud(pudp, __pud(0));
 
 	/*
 	 * Pentium-II erratum A13: in PAE mode we explicitly have to flush
 	 * the TLB via cr3 if the top-level pgd is changed...
 	 *
-	 * XXX I don't think we need to worry about this here, since
-	 * when clearing the pud, the calling code needs to flush the
-	 * tlb anyway.  But do it now for safety's sake. - jsgf
+	 * Make sure the pud entry we're updating is within the
+	 * current pgd to avoid unnecessary TLB flushes.
 	 */
-	write_cr3(read_cr3());
+	pgd = read_cr3();
+	if (__pa(pudp) >= pgd && __pa(pudp) < (pgd + sizeof(pgd_t)*PTRS_PER_PGD))
+		write_cr3(pgd);
 }
 
 #define pud_page(pud) \