Re: [PATCH v4] self_tests/kvm: sync_regs test for diag318

From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Collin Walling <walling@linux.ibm.com>,
	kvm@vger.kernel.org, linux-kselftest@vger.kernel.org
Cc: frankja@linux.ibm.com, david@redhat.com, thuth@redhat.com,
	cohuck@redhat.com, pbonzini@redhat.com, imbrenda@linux.ibm.com
Subject: Re: [PATCH v4] self_tests/kvm: sync_regs test for diag318
Date: Mon, 7 Dec 2020 21:09:30 +0100	[thread overview]
Message-ID: <87bf5fb7-26bf-f757-1efa-d2a49bde5188@de.ibm.com> (raw)
In-Reply-To: <d5289faa-0422-5f16-9d84-cd6c1d627033@linux.ibm.com>

On 07.12.20 21:06, Collin Walling wrote:
> On 12/7/20 2:32 PM, Christian Borntraeger wrote:
>> On 07.12.20 16:41, Collin Walling wrote:
>>> The DIAGNOSE 0x0318 instruction, unique to s390x, is a privileged call
>>> that must be intercepted via SIE, handled in userspace, and the
>>> information set by the instruction is communicated back to KVM.
>>>
>>> To test the instruction interception, an ad-hoc handler is defined which
>>> simply has a VM execute the instruction and then userspace will extract
>>> the necessary info. The handler is defined such that the instruction
>>> invocation occurs only once. It is up to the caller to determine how the
>>> info returned by this handler should be used.
>>>
>>> The diag318 info is communicated from userspace to KVM via a sync_regs
>>> call. This is tested During a sync_regs test, where the diag318 info is
>>> requested via the handler, then the info is stored in the appropriate
>>> register in KVM via a sync registers call.
>>>
>>> If KVM does not support diag318, then the tests will print a message
>>> stating that diag318 was skipped, and the asserts will simply test
>>> against a value of 0.
>>>
>>> Signed-off-by: Collin Walling <walling@linux.ibm.com>
>>
>> Interestingly enough, this testcase actually trigger a bug:
>> While we gracefully handle this (no crash)
>> debugfs: Directory 'kvm-200206' with parent 's390dbf' already present!
>> is certainly not ideal....
>>
> 
> Odd... I wonder what triggered this behavior?
> 
> I run my tests with a simple command:
> 
> make summary=0 TARGETS=kvm kselftest
> 
> This must have something to do with spinning up another VM to get the
> diag318 data. I think if I have the sync_regs test call the diag handler
> first, and then have the sync regs create a VM, that might solve that
> issue...

Yes, the s390dbf code will try to create a file named kvm-%pid. With
2 VMs the 2nd one fails. Luckily the kvm will be created anyway and 
also the shutdown seems to be fine, still....

> 
> May I ask how you encountered this bug so I may replicate in on my end?

I just did
make TARGETS=kvm selftests

and then the error is on dmesg.