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

[Greg Kurz <groug@kaod.org>]

On Fri, 27 Jul 2018 10:18:14 +0200
Thomas Huth <thuth@redhat.com> wrote:

> On 07/27/2018 09:54 AM, Greg Kurz wrote:
> > On Fri, 27 Jul 2018 15:27:24 +1000
> > David Gibson <david@gibson.dropbear.id.au> 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 <groug@kaod.org>    
> >>
> >> 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.  
> 
> For a "real" test, I think you'd need a guest OS that is reacting to the
> hot plug events. So maybe this should rather be done in the avocado
> framework instead?
> 

The intent isn't a "real" test actually, but just to exercise the XICS-KVM
paths in QEMU that get called during CPU hotplug. This cannot be achieved
with the qtest accelerator.

This patch would have revealed the regression in b585395b655a right away,
with the simple 'make check' developers are expected to run before posting.

Autotest has CPU hotplug tests, I don't know if avocado also has some, but
in any case, this is a different story IMHO.

Cheers,

--
Greg

>  Thomas
> 
>