Re: [Qemu-devel] [PATCH/RFC 4/5] s390x/kvm: test whether a cpu is STOPPED when checking "has_work"

[David Hildenbrand]

> > We have
> > - wait (wait bit in PSW)
> > - disabled wait (wait bit and interrupt fencing in PSW)
> > - STOPPED (not related to PSW, state change usually handled via service 
> > processor or hypervisor)
> >
> > I think we have to differentiate between KVM/TCG. On KVM we always do in 
> > kernel halt and qemu sees a halted only for STOPPED or disabled wait. TCG 
> > has to take care of the normal wait as well.
> >
> >  From a first glimpse, a disabled wait and STOPPED look similar, but there 
> > are (important) differences, e.g. other CPUs get a different a different 
> > result from a SIGP SENSE. This makes a big difference, e.g. for Linux 
> > guests, that send a SIGP STOP, followed by a SIGP SENSE loop until the CPU 
> > is down on hotplug (and shutdown, kexec..) So I think we agree, that 
> > handling the cpu states natively makes sense.
> >
> > The question is now only how to model it correctly without breaking TCG/KVM 
> > and reuse as much common code as possible. Correct?
> >
> > Do I understand you correctly, that your collapsing of stopped and halted 
> > is only in the qemu coding sense, IOW maybe we could just modify 
> > kvm_arch_process_async_events to consider the STOPPED state, as TCGs sigp 
> > implementation does not support SMP anyway?
> 
> That works for me, yes.
> 
> 
> Alex
> 

I had a look at it yesterday and it seems like we can totally drop this patch:

1. TCG doesn't support multiple CPUs and the TCG SIGP implementation isn't
ready for proper STOP/START/SENSE. Testing for STOPPED cpus in cpu_has_work()
can be dropped. To be able to support TCG was the main reason for this patch -
as we don't want to do so for now, we can leave it as is. We can still decide
to support the cpu states later using a mechanism suggest by Alex
(interrupt_requests).

Even if cpu_has_work() would make cpu.c:cpu_thread_is_idle() return false,
kvm_arch_process_async_events() called by kvm-all.c:kvm_cpu_exec() would make
it go back to sleep. Therefore a stopped VCPU will never be able to run in the
KVM case (because it always has cs->halted = true).

2. The unhalt in kvm_arch_process_async_events is for a special case where a
VCPU is in disabled wait and receives e.g. a machine-check interrupt. These
might happen in the future, for now we will never see them (the only
way to get a vcpu out of disabled wait are SIGP RESTART/CPU RESET - so we
don't break anything at that point).

David