Re: [Qemu-devel] [PATCH for-3.1] tests/cpu-plug-test: check CPU hotplug on ppc64 with KVM

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jul 30, 2018 at 11:59:00AM +0200, Greg Kurz wrote:
> On Mon, 30 Jul 2018 10:41:45 +0200
> Greg Kurz <groug@kaod.org> wrote:
> 
> > On Mon, 30 Jul 2018 15:57:15 +1000
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> [...]
> > > > > I'm pretty sure trying to change the accelerator on a qtest test just
> > > > > doesn't make sense.  We'd need a different approach for testing cpu
> > > > > hotplug against kvm & tcg backends.
> > > > >     
> > > > 
> > > > The test starts QEMU, triggers the CPU hotplug code with a QMP command
> > > > and checks the command didn't fail (or QEMU didn't crash, as it would
> > > > have before commit b585395b655a)... I really don't understand what
> > > > is wrong with that... Please elaborate.    
> > > 
> > > Well, ok, let me turn that around.  A test that doesn't rely on
> > > controlling the guest side behaviour at all probably shouldn't be a
> > > qtest based test, since that's what qtest is all about.
> > >   
> > 
> > The CPU hotplug test doesn't seem to do anything on the guest side: it
> > just checks that 'device_add' returns a response that isn't an error.
> > I'm not aware that the guest is expected to have a specific behavior
> > during 'device_add', apart from not crashing or hanging. That was the
> > initial idea behind passing '-S' to ensure the guest doesn't run.
> > 
> > Your remark seems to be more general though... are you meaning that
> > doing something like qtest_start("-machine accel=kvm:tcg") is just
> > wrong ?
> 
> The purpose of this test is simply to exercise a path in QEMU that
> is only used with KVM, but it can also be achieved the other way
> around:
> 
> @@ -189,7 +190,7 @@ static void xics_system_init(MachineState *machine, int nr_irqs, Error **errp)
>      sPAPRMachineState *spapr = SPAPR_MACHINE(machine);
>      Error *local_err = NULL;
>  
> -    if (kvm_enabled()) {
> +    if (kvm_enabled() || qtest_enabled()) {
>          if (machine_kernel_irqchip_allowed(machine) &&
>              !xics_kvm_init(spapr, &local_err)) {
> 
> This will test the setup of the in-kernel XICS when run on a book3s host,
> and fallback to emulated XICS otherwise (eg, travis).
> 
> Would this be more acceptable ?

No, I don't think that will work.  With this we call into kvm related
code via machine_kernel_irqchip_allowed() and xics_kvm_init() even in
the qtest case.  If they work on a host which doesn't have KVM (say
x86) it will only be by sheer accident.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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