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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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