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>

1 files changed, 15 insertions, 6 deletions

diff --git a/include/asm-x86/pgtable-3level.h b/include/asm-x86/pgtable-3level.h
index ed4c6f0e57ec..a195c3e757b9 100644
--- a/include/asm-x86/pgtable-3level.h
+++ b/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) \