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.