linux-kernel - Re: [PATCH 4/4] Enabling Access bit when doing memory swapping

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Message-ID: <20120518022241.GA14096@amt.cnet>
Date:	Thu, 17 May 2012 23:22:41 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xudong Hao <xudong.hao@...ux.intel.com>
Cc:	avi@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	haitao.shan@...el.com, xiantao.zhang@...el.com,
	xudong.hao@...el.com
Subject: Re: [PATCH 4/4] Enabling Access bit when doing memory swapping

On Wed, May 16, 2012 at 09:12:30AM +0800, Xudong Hao wrote:
> Enabling Access bit when doing memory swapping.
> 
> Signed-off-by: Haitao Shan <haitao.shan@...el.com>
> Signed-off-by: Xudong Hao <xudong.hao@...el.com>
> ---
>  arch/x86/kvm/mmu.c |   13 +++++++------
>  arch/x86/kvm/vmx.c |    6 ++++--
>  2 files changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index ff053ca..5f55f98 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1166,7 +1166,8 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
>     int young = 0;
> 
>     /*
> -    * Emulate the accessed bit for EPT, by checking if this page has
> +    * In case of absence of EPT Access and Dirty Bits supports,
> +    * emulate the accessed bit for EPT, by checking if this page has
>      * an EPT mapping, and clearing it if it does. On the next access,
>      * a new EPT mapping will be established.
>      * This has some overhead, but not as much as the cost of swapping
> @@ -1179,11 +1180,11 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
>     while (spte) {
>         int _young;
>         u64 _spte = *spte;
> -       BUG_ON(!(_spte & PT_PRESENT_MASK));
> -       _young = _spte & PT_ACCESSED_MASK;
> +       BUG_ON(!is_shadow_present_pte(_spte));
> +       _young = _spte & shadow_accessed_mask;
>         if (_young) {
>             young = 1;
> -           clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
> +           *spte &= ~shadow_accessed_mask;
>         }

Now a dirty bit can be lost. Is there a reason to remove the clear_bit?

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