linux-kernel - Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in get_dirty

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Message-Id: <20120302141134.1971c5da.yoshikawa.takuya@oss.ntt.co.jp>
Date:	Fri, 2 Mar 2012 14:11:34 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
Cc:	avi@...hat.com, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in
 get_dirty_log()

Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp> wrote:

> Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp> wrote:
> 
> > +	while (mask) {
> > +		rmapp = &slot->rmap[gfn_offset + __ffs(mask)];
> > +		__rmap_write_protect(kvm, rmapp, PT_PAGE_TABLE_LEVEL);
> >  
> > -	return write_protected;
> > +		/* clear the first set bit */
> > +		mask &= mask - 1;
> > +	}
> >  }
> 
> while (mask) {
> 	fsbit = __ffs(mask);
> 	gfn_offset += fsbit;
> 	mask >>= fsbit + 1;
> 
> 	rmapp = &slot->rmap[gfn_offset];
> 	__rmap_write_protect(kvm, rmapp, PT_PAGE_TABLE_LEVEL);
> }
> 
> Seems to be better: BSF friendly.

I was wrong.

"and < shift" and bsf did not show any difference for this kind of loop
on my box.  So we should keep the original implementation.

	Takuya
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