[Qemu-devel] Re: Role of qemu_fair_mutex

[Avi Kivity]

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.

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

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