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SubjectRE: [PATCH 4/4] Enabling Access bit when doing memory swapping
Date
> -----Original Message-----
> From: Marcelo Tosatti [mailto:mtosatti@redhat.com]
> Sent: Friday, May 18, 2012 10:23 AM
> To: Xudong Hao
> Cc: avi@redhat.com; kvm@vger.kernel.org; linux-kernel@vger.kernel.org;
> Shan, Haitao; Zhang, Xiantao; Hao, Xudong
> 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@intel.com>
> > Signed-off-by: Xudong Hao <xudong.hao@intel.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?

The clear_bit() is called in shadown and EPT A/D mode, because PT_ACCESSED_SHIFT does not make sense for EPT A/D bit, so use the code shadow_accessed_mask to mask the bit for both of them.


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