Re: [PATCH v2] KVM: MMU: Don't use RCU for lockless shadow walking

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 04/27/2012 06:00 AM, Marcelo Tosatti wrote:


>>  static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
>>  {
>> -	/* Decrease the counter after walking shadow page table finished */
>> -	smp_mb__before_atomic_dec();
>> -	atomic_dec(&vcpu->kvm->arch.reader_counter);
>> -	rcu_read_unlock();
>> +	/*
>> +	 * Make our reads and writes to shadow page tables globally visible
>> +	 * before leaving READING_SHADOW_PAGE_TABLES mode.
>> +	 */
> 
> This comment is misleading. Writes to shadow page tables must be
> performed with locked instructions outside the mmu_lock.
> 


You mean that the write should guarantee a correct memory order by itself?

>> +	smp_mb();
>> +	vcpu->mode = OUTSIDE_GUEST_MODE;
> 
> Don't you want 
> 
> vcpu->mode = OUTSIDE_GUEST_MODE;
> smp_mb();
> 


It is unsafe i think, it is a problem if spte read / spte update is ordered
to the behind of vcpu->mode = OUTSIDE_GUEST_MODE, like below:

VCPU 0                                  VCPU 1
                                   commit_zapped_page:
/*
 * setting vcpu->mode is reordered
 * to the head of read spte.
 */
vcpu->mode = OUTSIDE_GUEST_MODE;

                                   see VCPU 0 is out-of-guest-mode, IPI is
                                   not sent, and the sp is free immediately.

read spte;
 OOPS!!!

(It is invalid since spte is freed.)

smp_mb