linux-kernel - Re: [PATCH v3 1/2] KVM: selftests: Sync data verify of dirty logging with guest sync

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Message-ID: <e110673a-8422-bdff-4336-bdb486842d39@redhat.com>
Date:   Tue, 20 Apr 2021 16:07:51 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Andrew Jones <drjones@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v3 1/2] KVM: selftests: Sync data verify of dirty logging
 with guest sync

On 20/04/21 15:10, Peter Xu wrote:
> On Tue, Apr 20, 2021 at 10:07:16AM +0200, Paolo Bonzini wrote:
>> On 18/04/21 14:43, Peter Xu wrote:
>>> ----8<-----
>>> diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
>>> index 25230e799bc4..d3050d1c2cd0 100644
>>> --- a/tools/testing/selftests/kvm/dirty_log_test.c
>>> +++ b/tools/testing/selftests/kvm/dirty_log_test.c
>>> @@ -377,7 +377,7 @@ static void dirty_ring_after_vcpu_run(struct kvm_vm *vm, int ret, int err)
>>>           /* A ucall-sync or ring-full event is allowed */
>>>           if (get_ucall(vm, VCPU_ID, NULL) == UCALL_SYNC) {
>>>                   /* We should allow this to continue */
>>> -               ;
>>> +               vcpu_handle_sync_stop();
>>>           } else if (run->exit_reason == KVM_EXIT_DIRTY_RING_FULL ||
>>>                      (ret == -1 && err == EINTR)) {
>>>                   /* Update the flag first before pause */
>>> ----8<-----
>>>
>>> That's my intention when I introduce vcpu_handle_sync_stop(), but forgot to
>>> add...
>>
>> And possibly even this (untested though):
>>
>> diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
>> index ffa4e2791926..918954f01cef 100644
>> --- a/tools/testing/selftests/kvm/dirty_log_test.c
>> +++ b/tools/testing/selftests/kvm/dirty_log_test.c
>> @@ -383,6 +383,7 @@ static void dirty_ring_after_vcpu_run(struct kvm_vm *vm, int ret, int err)
>>   		/* Update the flag first before pause */
>>   		WRITE_ONCE(dirty_ring_vcpu_ring_full,
>>   			   run->exit_reason == KVM_EXIT_DIRTY_RING_FULL);
>> +		atomic_set(&vcpu_sync_stop_requested, false);
>>   		sem_post(&sem_vcpu_stop);
>>   		pr_info("vcpu stops because %s...\n",
>>   			dirty_ring_vcpu_ring_full ?
>> @@ -804,8 +805,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
>>   		 * the flush of the last page, and since we handle the last
>>   		 * page specially verification will succeed anyway.
>>   		 */
>> -		assert(host_log_mode == LOG_MODE_DIRTY_RING ||
>> -		       atomic_read(&vcpu_sync_stop_requested) == false);
>> +		assert(atomic_read(&vcpu_sync_stop_requested) == false);
>>   		vm_dirty_log_verify(mode, bmap);
>>   		sem_post(&sem_vcpu_cont);
>>
>> You can submit all these as a separate patch.
> 
> But it could race, then?
> 
>          main thread                 vcpu thread
>          -----------                 -----------
>                                    ring full
>                                      vcpu_sync_stop_requested=0
>                                      sem_post(&sem_vcpu_stop)
>       vcpu_sync_stop_requested=1
>       sem_wait(&sem_vcpu_stop)
>       assert(vcpu_sync_stop_requested==0)   <----

Yes, it could indeed.

Thanks,

Paolo