Re: [Qemu-devel] [PATCH] cpu: smp_wmb before lauching cpus.

[Jan Kiszka]

On 2012-07-06 09:46, liu ping fan wrote:
> On Thu, Jul 5, 2012 at 7:58 PM, Jan Kiszka <address@hidden> wrote:
>> On 2012-07-05 13:02, liu ping fan wrote:
>>> On Thu, Jul 5, 2012 at 6:16 PM, Jan Kiszka <address@hidden> wrote:
>>>> On 2012-07-05 12:10, liu ping fan wrote:
>>>>> On Thu, Jul 5, 2012 at 2:46 PM, Jan Kiszka <address@hidden> wrote:
>>>>>> On 2012-07-05 04:18, Liu Ping Fan wrote:
>>>>>>> Vcpu state must be set completely before receiving INIT-IPI,SIPI
>>>>>>>
>>>>>>> Signed-off-by: Liu Ping Fan <address@hidden>
>>>>>>> ---
>>>>>>>  kvm.h |    1 +
>>>>>>>  1 files changed, 1 insertions(+), 0 deletions(-)
>>>>>>>
>>>>>>> diff --git a/kvm.h b/kvm.h
>>>>>>> index 9c7b0ea..5b3c228 100644
>>>>>>> --- a/kvm.h
>>>>>>> +++ b/kvm.h
>>>>>>> @@ -198,6 +198,7 @@ static inline void 
>>>>>>> cpu_synchronize_post_init(CPUArchState *env)
>>>>>>>  {
>>>>>>>      if (kvm_enabled()) {
>>>>>>>          kvm_cpu_synchronize_post_init(env);
>>>>>>> +        smp_wmb();
>>>>>>>      }
>>>>>>>  }
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> In theory, there should be no vcpu kick-off after this without some
>>>>>> locking operations involved that imply barriers. Did you see real
>>>>>
>>>>> Yeah, but what if it is non-x86?
>>>>
>>>> The locking I'm referring to is arch independent.
>>>>
>>>>>> inconsistencies without this explicit one?
>>>>
>>>> Again: Did you see real issues or is this based on static analysis?
>>>>
>>> Just on  static analysis
>>
>> Then please describe - also for the changelog - at least one case in
>> details where this is needed.
>>
> I dived into code. And yes, as you said, ACPI eject does involve some
> locking operation. So the only thing left is for starting up.  There
> seems no potential lock operation from cpu_synchronize_all_post_init()
> to  resume_all_vcpus().

There is no difference in this scenario as well: the vcpu is waiting on
a condvar for the initial kick-off and will synchronize with the main
thread via the global mutex. When in doubt, try using a debugger.

Jan

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