[Qemu-devel] Re: Role of qemu_fair_mutex

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/03/2011 04:01 AM, Avi Kivity wrote:
> On 01/03/2011 11:46 AM, Jan Kiszka wrote:
>> Hi,
>>
>> at least in kvm mode, the qemu_fair_mutex seems to have lost its
>> function of balancing qemu_global_mutex access between the io-thread and
>> vcpus. It's now only taken by the latter, isn't it?
>>
>> This and the fact that qemu-kvm does not use this kind of lock made me
>> wonder what its role is and if it is still relevant in practice. I'd
>> like to unify the execution models of qemu-kvm and qemu, and this lock
>> is the most obvious difference (there are surely more subtle ones as
>> well...).
>>
>
> IIRC it was used for tcg, which has a problem that kvm doesn't have: a 
> tcg vcpu needs to hold qemu_mutex when it runs, which means there will 
> always be contention on qemu_mutex.  In the absence of fairness, the 
> tcg thread could dominate qemu_mutex and starve the iothread.

No, it's actually the opposite IIRC.

TCG relies on the following behavior.   A guest VCPU runs until 1) it 
encounters a HLT instruction 2) an event occurs that forces the TCG 
execution to break.

(2) really means that the TCG thread receives a signal.  Usually, this 
is the periodic timer signal.

When the TCG thread, it needs to let the IO thread run for at least one 
iteration.  Coordinating the execution of the IO thread such that it's 
guaranteed to run at least once and then having it drop the qemu mutex 
long enough for the TCG thread to acquire it is the purpose of the 
qemu_fair_mutex.

Regards,

Anthony Liguori

> This doesn't happen with kvm since kvm vcpus drop qemu_mutex when 
> running.