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Date: Wed, 6 Jul 2011 13:51:34 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: Avi Kivity <avi@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        KVM <kvm@vger.kernel.org>
Subject: Re: [PATCH v3 05/19] KVM: MMU: optimize to handle dirty bit
Message-ID: <20110706165134.GC24417@amt.cnet>
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 <20110705192703.GB24417@amt.cnet>
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On Wed, Jul 06, 2011 at 09:22:28AM +0800, Xiao Guangrong wrote:
> On 07/06/2011 03:27 AM, Marcelo Tosatti wrote:
> > On Thu, Jun 30, 2011 at 04:21:30PM +0800, Xiao Guangrong wrote:
> >> If dirty bit is not set, we can make the pte access read-only to avoid handing
> >> dirty bit everywhere
> >>
> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> > 
> >> -static unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, pt_element_t gpte)
> >> +static unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, pt_element_t gpte,
> >> +				   bool last)
> >>  {
> >>  	unsigned access;
> >>  
> >>  	access = (gpte & (PT_WRITABLE_MASK | PT_USER_MASK)) | ACC_EXEC_MASK;
> >> +	if (last && !is_dirty_gpte(gpte))
> >> +		access &= ~ACC_WRITE_MASK;
> >> +
> > 
> > What if the walker marks the dirty bit on the gpte?
> > 
> 
> Actually, we get guest pte access after mark the dirty bit:
> 
> 	if (write_fault && unlikely(!is_dirty_gpte(pte))) {
> 		int ret;
> 
> 		trace_kvm_mmu_set_dirty_bit(table_gfn, index, sizeof(pte));
> 		ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index,
> 					  pte, pte|PT_DIRTY_MASK);
> 		if (unlikely(ret < 0)) {
> 			present = false;
> 			goto error;
> 		} else if (ret)
> 			goto walk;
> 
> 		mark_page_dirty(vcpu->kvm, table_gfn);
> 		pte |= PT_DIRTY_MASK;
> 		walker->ptes[walker->level - 1] = pte;
> 	}
> 
> 	pte_access = pt_access & FNAME(gpte_access)(vcpu, pte, true);
>  
> So, i think it works well :-)

I think you meant after marking the accessed bit. The dirty bit is set
just before returning.