x86: defer cr3 reload when doing pud_clear()

PAE mode requires that we reload cr3 in order to guarantee that
changes to the pgd will be noticed by the processor.  This means that
in principle pud_clear needs to reload cr3 every time.  However,
because reloading cr3 implies a tlb flush, we want to avoid it where
possible.

pud_clear() is only used in a couple of places:
 - in free_pmd_range(), when pulling down a range of process address space, and
 - huge_pmd_unshare()

In both cases, the calling code will do a a tlb flush anyway, so
there's no need to do it within pud_clear().

In free_pmd_range(), the pud_clear is immediately followed by
pmd_free_tlb(); we can hook that to make the mmu_gather do an
unconditional full flush to make sure cr3 gets reloaded.

In huge_pmd_unshare, it is followed by flush_tlb_range, which always
results in a full cr3-reload tlb flush.
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: William Irwin <wli@holomorphy.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/asm-x86/pgalloc_32.h

@@ -74,6 +74,13 @@ static inline void pmd_free(pmd_t *pmd)
 
 static inline void __pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd)
 {
+	/* This is called just after the pmd has been detached from
+	   the pgd, which requires a full tlb flush to be recognized
+	   by the CPU.  Rather than incurring multiple tlb flushes
+	   while the address space is being pulled down, make the tlb
+	   gathering machinery do a full flush when we're done. */
+	tlb->fullmm = 1;
+
 	paravirt_release_pd(__pa(pmd) >> PAGE_SHIFT);
 	tlb_remove_page(tlb, virt_to_page(pmd));
 }

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

@@ -96,14 +96,23 @@ static inline void pud_clear(pud_t *pudp)
 	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...
+	 * In principle we need to do a cr3 reload here to make sure
+	 * the processor recognizes the changed pgd.  In practice, all
+	 * the places where pud_clear() gets called are followed by
+	 * full tlb flushes anyway, so we can defer the cost here.
 	 *
-	 * 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
+	 * Specifically:
+	 *
+	 * mm/memory.c:free_pmd_range() - immediately after the
+	 * pud_clear() it does a pmd_free_tlb().  We change the
+	 * mmu_gather structure to do a full tlb flush (which has the
+	 * effect of reloading cr3) when the pagetable free is
+	 * complete.
+	 *
+	 * arch/x86/mm/hugetlbpage.c:huge_pmd_unshare() - the call to
+	 * this is followed by a flush_tlb_range, which on x86 does a
+	 * full tlb flush.
 	 */
-	write_cr3(read_cr3());
 }
 
 #define pud_page(pud) \