Re: [kvm-unit-tests PATCH] s390x: Workaround smp stop and store status race

[Janosch Frank <frankja@linux.ibm.com>]

On 2/9/21 5:19 PM, Thomas Huth wrote:
> On 09/02/2021 17.14, Janosch Frank wrote:
>> On 2/9/21 5:08 PM, Claudio Imbrenda wrote:
>>> On Tue,  9 Feb 2021 09:15:54 -0500
>>> Janosch Frank <frankja@linux.ibm.com> wrote:
>>>
>>>> KVM and QEMU handle a SIGP stop and store status in two steps:
>>>> 1) Stop the CPU by injecting a stop request
>>>> 2) Store when the CPU has left SIE because of the stop request
>>>>
>>>> The problem is that the SIGP order is already considered completed by
>>>> KVM/QEMU when step 1 has been performed and not once both have
>>>> completed. In addition we currently don't implement the busy CC so a
>>>> kernel has no way of knowing that the store has finished other than
>>>> checking the location for the store.
>>>>
>>>> This workaround is based on the fact that for a new SIE entry (via the
>>>> added smp restart) a stop with the store status has to be finished
>>>> first.
>>>>
>>>> Correct handling of this in KVM/QEMU will need some thought and time.
>>>
>>> do I understand correctly that you are here "fixing" the test by not
>>> triggering the KVM bug? Shouldn't we try to trigger as many bugs as
>>> possible instead?
>>
>> This is not a bug, it's missing code :-)
>>
>> We trigger a higher number of bugs by running tests and this workaround
>> does exactly that by letting Thomas use the smp test in the CI again.
> 
> Alternatively, we could use report_xfail here to make the test pass, but 
> still have the problem reported so that we do not forget to fix it later.
> 
>   Thomas
> 

I have no strong opinion on that although I'll need to have a look if
our CI flags XPASS/XFAIL as fails.