Re: [kvm-unit-tests PATCH v2 3/3] s390x: Ultravisor guest API test

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

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On 7/31/20 10:42 AM, Cornelia Huck wrote:
> On Fri, 31 Jul 2020 09:34:41 +0200
> Janosch Frank <frankja@linux.ibm.com> wrote:
> 
>> On 7/30/20 5:58 PM, Thomas Huth wrote:
>>> On 30/07/2020 13.16, Cornelia Huck wrote:  
>>>> On Mon, 27 Jul 2020 05:54:15 -0400
>>>> Janosch Frank <frankja@linux.ibm.com> wrote:
>>>>  
>>>>> Test the error conditions of guest 2 Ultravisor calls, namely:
>>>>>      * Query Ultravisor information
>>>>>      * Set shared access
>>>>>      * Remove shared access
>>>>>
>>>>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>>>>> Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
>>>>> ---
>>>>>  lib/s390x/asm/uv.h  |  68 +++++++++++++++++++
>>>>>  s390x/Makefile      |   1 +
>>>>>  s390x/unittests.cfg |   3 +
>>>>>  s390x/uv-guest.c    | 159 ++++++++++++++++++++++++++++++++++++++++++++
>>>>>  4 files changed, 231 insertions(+)
>>>>>  create mode 100644 lib/s390x/asm/uv.h
>>>>>  create mode 100644 s390x/uv-guest.c
>>>>>  
>>>>
>>>> (...)
>>>>  
>>>>> +static inline int uv_call(unsigned long r1, unsigned long r2)
>>>>> +{
>>>>> +	int cc;
>>>>> +
>>>>> +	asm volatile(
>>>>> +		"0:	.insn rrf,0xB9A40000,%[r1],%[r2],0,0\n"
>>>>> +		"		brc	3,0b\n"
>>>>> +		"		ipm	%[cc]\n"
>>>>> +		"		srl	%[cc],28\n"
>>>>> +		: [cc] "=d" (cc)
>>>>> +		: [r1] "a" (r1), [r2] "a" (r2)
>>>>> +		: "memory", "cc");
>>>>> +	return cc;
>>>>> +}  
>>>>
>>>> This returns the condition code, but no caller seems to check it
>>>> (instead, they look at header.rc, which is presumably only set if the
>>>> instruction executed successfully in some way?)
>>>>
>>>> Looking at the kernel, it retries for cc > 1 (presumably busy
>>>> conditions), and cc != 0 seems to be considered a failure. Do we want
>>>> to look at the cc here as well?  
>>>
>>> It's there - but here it's in the assembly code, the "brc 3,0b".  
> 
> Ah yes, I missed that.
> 
>>
>> Yes, we needed to factor that out in KVM because we sometimes need to
>> schedule and then it looks nicer handling that in C code. The branch on
>> condition will jump back for cc 2 and 3. cc 0 and 1 are success and
>> error respectively and only then the rc and rrc in the UV header are set.
> 
> Yeah, it's a bit surprising that rc/rrc are also set with cc 1.

Is it?
The (r)rc *only* contain meaningful information on CC 1.
On CC 0 they will simply say everything is fine which CC 0 states
already anyway.

> 
> (Can you add a comment? Just so that it is clear that callers never
> need to check the cc, as rc/rrc already contain more information than
> that.)

I'd rather fix my test code and also check the CC.
I did check it for my other UV tests so I've no idea why I didn't do it
here...


How about adding a comment for the cc 2/3 case?
"The brc instruction will take care of the cc 2/3 case where we need to
continue the execution because we were interrupted.
The inline assembly will only return on success/error i.e. cc 0/1."

> 
>>
>>>
>>> Patch looks ok to me (but I didn't do a full review):
>>>
>>> Acked-by: Thomas Huth <thuth@redhat.com>
>>>   
>>
>>
> 



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