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

Xiao Guangrong
2011-07-06T01:20:33Z

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 :-)