Re: [XEN PATCH][for-4.19 v5] xen: Add deviations for MISRA C:2012 Rule 7.1

[Luca Fancellu <Luca.Fancellu@xxxxxxx>]

> On 31 Oct 2023, at 15:27, Julien Grall <julien@xxxxxxx> wrote:
> 
> Hi,
> 
> On 31/10/2023 15:12, Luca Fancellu wrote:
>>> On 31 Oct 2023, at 15:10, Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx> wrote:
>>> 
>>> On 2023-10-31 15:13, Luca Fancellu wrote:
>>>>> On 31 Oct 2023, at 13:27, Julien Grall <julien@xxxxxxx> wrote:
>>>>> Hi Stefano,
>>>>> On 30/10/2023 22:49, Stefano Stabellini wrote:
>>>>>> On Mon, 30 Oct 2023, Julien Grall wrote:
>>>>>>> Hi Nicola,
>>>>>>> On 27/10/2023 16:11, Nicola Vetrini wrote:
>>>>>>>> diff --git a/docs/misra/deviations.rst b/docs/misra/deviations.rst
>>>>>>>> index 8511a189253b..8aaaa1473fb4 100644
>>>>>>>> --- a/docs/misra/deviations.rst
>>>>>>>> +++ b/docs/misra/deviations.rst
>>>>>>>> @@ -90,6 +90,13 @@ Deviations related to MISRA C:2012 Rules:
>>>>>>>>           - __emulate_2op and __emulate_2op_nobyte
>>>>>>>>           - read_debugreg and write_debugreg
>>>>>>>>  +   * - R7.1
>>>>>>>> +     - It is safe to use certain octal constants the way they are 
>>>>>>>> defined
>>>>>>>> +       in specifications, manuals, and algorithm descriptions. Such 
>>>>>>>> places
>>>>>>>> +       are marked safe with a /\* octal-ok \*/ in-code comment, or 
>>>>>>>> with a
>>>>>>>> SAF
>>>>>>>> +       comment (see safe.json).
>>>>>>> Reading this, it is unclear to me why we have two ways to deviate the 
>>>>>>> rule
>>>>>>> r7.1. And more importantely, how would the developper decide which one 
>>>>>>> to use?
>>>>>> I agree with you on this and we were discussing this topic just this
>>>>>> morning in the FUSA community call. I think we need a way to do this
>>>>>> with the SAF framework:
>>>>>> if (some code with violation) /* SAF-xx-safe */
>>>>>> This doesn't work today unfortunately. It can only be done this way:
>>>>>> /* SAF-xx-safe */
>>>>>> if (some code with violation)
>>>>>> Which is not always desirable. octal-ok is just an ad-hoc solution for
>>>>>> one specific violation but we need a generic way to do this. Luca is
>>>>>> investigating possible ways to support the previous format in SAF.
>>>>> Why can't we use octal-ok everywhere for now? My point here is to make 
>>>>> simple for the developper to know what to use.
>>>>>> I think we should take this patch for now and harmonize it once SAF is
>>>>>> improved.
>>>>> The description of the deviation needs some improvement. To give an 
>>>>> example, with the current wording, one could they can use octal-ok 
>>>>> everywhere. But above, you are implying that SAF-xx-safe should be
>>>>> preferred.
>>>>> I would still strongly prefer if we use octal-ok everywhere because this 
>>>>> is simple to remember. But if the other are happy to have both SAF-XX and 
>>>>> octal-ok, then the description needs to be completely unambiguous and the 
>>>>> patch should contain some explanation why we have two different ways to 
>>>>> deviate.
>>>> Would it be ok to have both, for example: /* SAF-XX-safe octal-ok */
>>>> So that the suppression engine do what it should (currently it doesn’t 
>>>> suppress the same line, but we could do something about it) and the 
>>>> developer
>>>> has a way to understand what is the violation here without going to the 
>>>> justification database.
>>> 
>>> I guess. It could overflow the 80-char limit in xen/arch/x86/hvm/svm/svm.h, 
>>> though.
>> Yeah, but we could rule out something in code_style to allow only this kind 
>> of trailing comments to exceed the 80 chars
> 
> In the past I expressed concerned with this kind of the rule because it is 
> not entirely clear how an automatic formatter will be able to check it.
> 
> Can you clarify whether clang-format would be able to handle your proposed 
> rule?

So, yesterday Bertrand pointed out a StackOverflow thread for this issue and if 
we use ReflowComments: false we should
be able to let the line as it is (not tested).

https://clang.llvm.org/docs/ClangFormatStyleOptions.html#reflowcomments



> 
> Cheers,
> 
> -- 
> Julien Grall