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

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

On 12/7/20 9:59 PM, Collin Walling wrote:
> On 12/7/20 3:16 PM, Christian Borntraeger wrote:
>>
>>
>> On 07.12.20 21:13, Collin Walling wrote:
>>> On 12/7/20 3:09 PM, Christian Borntraeger wrote:
>>>>
>>>>
>>>> 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.
>>>>
>>>
>>> Thanks. v5 with fix incoming.
>>
>> I think the test is actually fine and we should rather fix the kvm module to
>> gracefully handle a userspace that starts up 2 or more guests.
>>
> 
> Looks like the root cause is within the inode.c file used for the debug
> filesystem. Essentially, the 2nd VM starts / ends just fine as you
> mentioned, but doesn't get a dbfs.
> 
> Since this touches the dbfs related area, and I'm unsure how common this
> problem is out in the wild, should we ping the kernel list and see if it
> catches anyone's attention?
> 
> A first thought would be to append a per-userspace incrementing value to
> the end of the kvm-%pid part to account for any collisions, but that's
> up to the folks that know more than I do.
> 

Seems like I have a deja-vu, I remember having to fix that for the
common code :-)
For the KVM debugfs I solved that by appending the file descriptor after
the pid. Maybe we can steal the string from the dentry.

Have a look into vm_create_vm_debugfs(struct kvm *kvm, int fd).