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

Xiao Guangrong
2011-07-06T19:10:37Z

On 07/07/2011 12:51 AM, Marcelo Tosatti wrote:
> 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.
>

In this patch, i moved getting pte_access to the behind of setting dirty bit set