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

[Thomas Huth]

On 07/27/2018 11:00 AM, Greg Kurz wrote:
> On Fri, 27 Jul 2018 10:18:14 +0200
> Thomas Huth <address@hidden> wrote:
> 
>> On 07/27/2018 09:54 AM, 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.  
>>
>> 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.

OK, that's fair. I think what rather bugs me here is that you start QEMU
with -S to work around something that sounds like real bug in SLOF ...
any chance that you could fix that bug in SLOF instead of using -S here?

 Thomas