RE: [PATCH 4/4] Enabling Access bit when doing memory swapping

[Hao, Xudong]

> -----Original Message-----
> From: Avi Kivity [mailto:avi@xxxxxxxxxx]
> Sent: Monday, May 21, 2012 4:32 PM
> To: Hao, Xudong
> Cc: Marcelo Tosatti; Xudong Hao; kvm@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Shan, Haitao; Zhang, Xiantao
> Subject: Re: [PATCH 4/4] Enabling Access bit when doing memory swapping
> 
> On 05/21/2012 06:22 AM, Hao, Xudong wrote:
> > > -----Original Message-----
> > > From: Marcelo Tosatti [mailto:mtosatti@xxxxxxxxxx]
> > > Sent: Friday, May 18, 2012 10:23 AM
> > > To: Xudong Hao
> > > Cc: avi@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > 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@xxxxxxxxx>
> > > > Signed-off-by: Xudong Hao <xudong.hao@xxxxxxxxx>
> > > > ---
> > > >  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.
> 
> That doesn't answer the question.  An atomic operation is now non-atomic.
> 
> You can calculate shadow_accessed_bit and keep on using clear_bit(), or
> switch to cmpxchg64(), but don't just drop the dirty bit here.
> 

I know your meaning. How about this changes:

...
            young = 1;
+            if (enable_ept_ad_bits)
+                clear_bit(ffs(shadow_accessed_mask), (unsigned long *)spte);
            clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
...

> --
> error compiling committee.c: too many arguments to function

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