linux-kernel - Re: [PATCH v3 05/19] KVM: MMU: optimize to handle dirty bit

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Date:	Wed, 06 Jul 2011 09:22:28 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 05/19] KVM: MMU: optimize to handle dirty bit

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@...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 :-)
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