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

[Greg Kurz]

On Mon, 30 Jul 2018 15:57:15 +1000
David Gibson <address@hidden> wrote:

> On Fri, Jul 27, 2018 at 09:54:52AM +0200, Greg Kurz wrote:
> > On Fri, 27 Jul 2018 15:27:24 +1000
> > David Gibson <address@hidden> wrote:
> >   
> > > On Wed, Jul 25, 2018 at 04:45:26PM +0200, Greg Kurz wrote:  
> > > > Commit b585395b655 fixed a regression introduced by some recent changes
> > > > in the XICS code, that was causing QEMU to crash instantly during CPU
> > > > hotplug with KVM. This is typically the kind of bug we'd like our
> > > > test suite to detect before it gets merged. Unfortunately, the current
> > > > tests run with '-machine accel=qtest' and don't exercise KVM specific
> > > > paths in QEMU.
> > > > 
> > > > This patch hence changes add_pseries_test_case() to launch QEMU with
> > > > '-machine accel=kvm' if KVM is available.
> > > > 
> > > > A notable consequence is that the guest will execute SLOF, but for some
> > > > reasons SLOF sometimes hits a program exception. This causes the guest
> > > > to loop forever and the test to be stuck.  Since we don't need the guest
> > > > to be truely running, let's pass -S to QEMU to avoid that.
> > > > 
> > > > Also disable machine capabilities that could be unavailable in KVM, eg,
> > > > when using PR KVM.
> > > > 
> > > > Signed-off-by: Greg Kurz <address@hidden>    
> > > 
> > > 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 ?

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