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    SubjectRe: [PATCH 1/4] KVM: MMU: don't drop spte if overwrite it from W to RO
    On Fri, Nov 12, 2010 at 06:30:22PM +0800, Xiao Guangrong wrote:
    > We just need flush tlb if overwrite a writable spte with a read-only one
    >
    > Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
    > ---
    > arch/x86/kvm/mmu.c | 19 +++++++++----------
    > 1 files changed, 9 insertions(+), 10 deletions(-)
    >
    > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
    > index 4b6d54c..1a93ab4 100644
    > --- a/arch/x86/kvm/mmu.c
    > +++ b/arch/x86/kvm/mmu.c
    > @@ -2044,6 +2044,15 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
    > if (pte_access & ACC_WRITE_MASK)
    > mark_page_dirty(vcpu->kvm, gfn);
    >
    > + /*
    > + * If we overwrite a writable spte with a read-only one,
    > + * flush remote TLBs. Otherwise rmap_write_protect will
    > + * find a read-only spte, even though the writable spte
    > + * might be cached on a CPU's TLB.
    > + */
    > + else if (is_writable_pte(*sptep))
    > + ret = 1;
    > +

    The return value of set_spte indicates whether the gfn being mapped to
    was write protected, not if a TLB flush is necessary.

    > set_pte:
    > update_spte(sptep, spte);
    > done:
    > @@ -2084,16 +2093,6 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
    > spte_to_pfn(*sptep), pfn);
    > drop_spte(vcpu->kvm, sptep, shadow_trap_nonpresent_pte);
    > kvm_flush_remote_tlbs(vcpu->kvm);
    > - /*
    > - * If we overwrite a writable spte with a read-only one,
    > - * drop it and flush remote TLBs. Otherwise rmap_write_protect
    > - * will find a read-only spte, even though the writable spte
    > - * might be cached on a CPU's TLB.
    > - */
    > - } else if (is_writable_pte(*sptep) &&
    > - (!(pte_access & ACC_WRITE_MASK) || !dirty)) {
    > - drop_spte(vcpu->kvm, sptep, shadow_trap_nonpresent_pte);
    > - kvm_flush_remote_tlbs(vcpu->kvm);
    > } else
    > was_rmapped = 1;

    And here, flush will only happen if overwrite is RW->RO.



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