linux-kernel - Re: [PATCH v2 2/6] KVM: MMU: don't drop spte if overwrite it from W to RO

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Date:	Wed, 17 Nov 2010 13:42:41 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 2/6] KVM: MMU: don't drop spte if overwrite it from W
 to RO

On Wed, Nov 17, 2010 at 12:10:50PM +0800, Xiao Guangrong wrote:
> We just need flush tlb if overwrite a writable spte with a read-only
> one.
> 
> And we should move this operation to set_spte() for sync_page path
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
> ---
>  arch/x86/kvm/mmu.c |   20 +++++++++-----------
>  1 files changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index e008ae7..9bad960 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1966,7 +1966,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  		    gfn_t gfn, pfn_t pfn, bool speculative,
>  		    bool can_unsync, bool reset_host_protection)
>  {
> -	u64 spte;
> +	u64 spte, entry = *sptep;
>  	int ret = 0;
>  
>  	/*
> @@ -2039,6 +2039,14 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  
>  set_pte:
>  	update_spte(sptep, spte);
> +	/*
> +	 * If we overwrite a writable spte with a read-only one we
> +	 * should 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.
> +	 */
> +	if (is_writable_pte(entry) && !is_writable_pte(*sptep))
> +		kvm_flush_remote_tlbs(vcpu->kvm);

There is no need to flush on sync_page path since the guest is
responsible for it.

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