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

[Avi Kivity <avi@redhat.com>]

On 05/21/2012 02:17 PM, Hao, Xudong wrote:
> > -----Original Message-----
> > From: Avi Kivity [mailto:avi@redhat.com]
> > Sent: Monday, May 21, 2012 6:48 PM
> > To: Hao, Xudong
> > Cc: Marcelo Tosatti; Xudong Hao; kvm@vger.kernel.org;
> > linux-kernel@vger.kernel.org; Shan, Haitao; Zhang, Xiantao
> > Subject: Re: [PATCH 4/4] Enabling Access bit when doing memory swapping
> > 
> > On 05/21/2012 01:35 PM, Hao, Xudong wrote:
> > > >
> > > > 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);
> > 
> > ffs() returns an off-by-one result, so this needs to be adjusted. 
>
> Yes, it need to decrease 1, I'll send v3 version for patch4, any other comments?

I think it's fine.

> > IIRC
> > bsfl is slow, but this shouldn't be a problem here.
> > 
>
> I do not know the story...
>

No story, bsf is a relatively slow instruction, but it shouldn't affect
us here; this isn't a fast path and in any case it's only a few cycles.

-- 
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