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

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

On 04/24/2012 05:19 PM, Avi Kivity wrote:


>>> Turned out to be simpler than expected.  However, I think there's a problem
>>> with make_all_cpus_request() possible reading an incorrect vcpu->cpu.
>>
>>
>> It seems possible.
>>
>> Can we fix it by reading vcpu->cpu when the vcpu is in GUEST_MODE or
>> EXITING_GUEST_MODE (IIRC, in these modes, interrupt is disabled)?
>>
>> Like:
>>
>> if (kvm_vcpu_exiting_guest_mode(vcpu) != OUTSIDE_GUEST_MODE)
>>                       cpumask_set_cpu(vcpu->cpu, cpus);
> 
> I think it is actually okay.  We are only vulnerable if lockless shadow
> walk started during prepare_zap_page(), and extends past
> kvm_flush_remote_tlbs(), yes?  But in that case, vcpu->cpu is stable
> since local_irq_disable() kills preemption.
> 


This case can happen?

   VCPU 0                                           VCPU 1

kvm_for_each_vcpu(i, vcpu, kvm) {
	kvm_make_request(req, vcpu);

                                                  VCPU1 is running on CPU 1 out of guest mode

	cpu = vcpu->cpu;

	/* Set ->requests bit before we read ->mode */
	smp_mb();

	if (cpus != NULL && cpu != -1 && cpu != me &&

						 VCPU1 is scheduled to CPU 2, and running in
                                                 guest mode

	      kvm_vcpu_exiting_guest_mode(vcpu) != OUTSIDE_GUEST_MODE)
		cpumask_set_cpu(cpu, cpus);
}

       VCPU 0 send IPI to CPU1, but actually, VCPU1 is running on CPU 2.