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

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

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.

> 
>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>> ---
>>  s390x/smp.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/s390x/smp.c b/s390x/smp.c
>> index b0ece491..32f284a2 100644
>> --- a/s390x/smp.c
>> +++ b/s390x/smp.c
>> @@ -102,12 +102,15 @@ static void test_stop_store_status(void)
>>  	lc->grs_sa[15] = 0;
>>  	smp_cpu_stop_store_status(1);
>>  	mb();
>> +	report(smp_cpu_stopped(1), "cpu stopped");
>> +	/* For the cpu to be started it should have finished storing
>> */
>> +	smp_cpu_restart(1);
>>  	report(lc->prefix_sa == (uint32_t)(uintptr_t)cpu->lowcore,
>> "prefix"); report(lc->grs_sa[15], "stack");
>> -	report(smp_cpu_stopped(1), "cpu stopped");
>>  	report_prefix_pop();
>>  
>>  	report_prefix_push("stopped");
>> +	smp_cpu_stop(1);
>>  	lc->prefix_sa = 0;
>>  	lc->grs_sa[15] = 0;
>>  	smp_cpu_stop_store_status(1);
>