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>
---
include/asm-x86/pgalloc_32.h | 7 +++++++
include/asm-x86/pgtable-3level.h | 21 +++++++++++++++------
2 files changed, 22 insertions(+), 6 deletions(-)
diff --git a/include/asm-x86/pgalloc_32.h b/include/asm-x86/pgalloc_32.h
--- a/include/asm-x86/pgalloc_32.h
+++ b/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));
}
diff --git a/include/asm-x86/pgtable-3level.h b/include/asm-x86/pgtable-3level.h
--- 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) \
--
