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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>
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2 changed files with 22 additions and 6 deletions
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@ -74,6 +74,13 @@ static inline void pmd_free(pmd_t *pmd)
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static inline void __pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd)
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{
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/* This is called just after the pmd has been detached from
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the pgd, which requires a full tlb flush to be recognized
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by the CPU. Rather than incurring multiple tlb flushes
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while the address space is being pulled down, make the tlb
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gathering machinery do a full flush when we're done. */
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tlb->fullmm = 1;
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paravirt_release_pd(__pa(pmd) >> PAGE_SHIFT);
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tlb_remove_page(tlb, virt_to_page(pmd));
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}
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@ -96,14 +96,23 @@ static inline void pud_clear(pud_t *pudp)
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set_pud(pudp, __pud(0));
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/*
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* Pentium-II erratum A13: in PAE mode we explicitly have to flush
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* the TLB via cr3 if the top-level pgd is changed...
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* In principle we need to do a cr3 reload here to make sure
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* the processor recognizes the changed pgd. In practice, all
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* the places where pud_clear() gets called are followed by
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* full tlb flushes anyway, so we can defer the cost here.
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*
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* XXX I don't think we need to worry about this here, since
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* when clearing the pud, the calling code needs to flush the
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* tlb anyway. But do it now for safety's sake. - jsgf
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* Specifically:
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*
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* mm/memory.c:free_pmd_range() - immediately after the
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* pud_clear() it does a pmd_free_tlb(). We change the
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* mmu_gather structure to do a full tlb flush (which has the
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* effect of reloading cr3) when the pagetable free is
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* complete.
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*
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* arch/x86/mm/hugetlbpage.c:huge_pmd_unshare() - the call to
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* this is followed by a flush_tlb_range, which on x86 does a
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* full tlb flush.
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*/
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write_cr3(read_cr3());
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}
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#define pud_page(pud) \
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