Re: [PATCH] misra: fix violations in macros GVA_INFO, TRACE_TIME

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2025-08-05 13:49, Dmytro Prokopchuk1 wrote:
> On 7/31/25 19:09, Nicola Vetrini wrote:
>> On 2025-07-31 18:05, Andrew Cooper wrote:
>>> On 31/07/2025 4:58 pm, Jan Beulich wrote:
>>>> On 31.07.2025 17:37, Andrew Cooper wrote:
>>>>> On 31/07/2025 4:16 pm, Dmytro Prokopchuk1 wrote:
>>>>>> MISRA Rule 13.1: Initializer lists shall not contain persistent 
>>>>>> side
>>>>>> effects.
>>>>>> 
>>>>>> The violations occur because both the `GVA_INFO` and `TRACE_TIME`
>>>>>> macro
>>>>>> expansions include expressions with persistent side effects 
>>>>>> introduced
>>>>>> via inline assembly.
>>>>>> 
>>>>>> In the case of `GVA_INFO`, the issue stems from the initializer 
>>>>>> list
>>>>>> containing a direct call to `current`, which evaluates to
>>>>>> `this_cpu(curr_vcpu)` and involves persistent side effects via the
>>>>>> `asm` statement. To resolve this, the side-effect-producing 
>>>>>> expression
>>>>>> is computed in a separate statement prior to the macro 
>>>>>> initialization:
>>>>>> 
>>>>>>     struct vcpu *current_vcpu = current;
>>>>>> 
>>>>>> The computed value is passed into the `GVA_INFO(current_vcpu)` 
>>>>>> macro,
>>>>>> ensuring that the initializer is clean and free of such side 
>>>>>> effects.
>>>>>> 
>>>>>> Similarly, the `TRACE_TIME` macro violates this rule when 
>>>>>> accessing
>>>>>> expressions like `current->vcpu_id` and 
>>>>>> `current->domain->domain_id`,
>>>>>> which also depend on `current` and inline assembly. To fix this, 
>>>>>> the
>>>>>> value of `current` is assigned to a temporary variable:
>>>>>> 
>>>>>>     struct vcpu *v = current;
>>>>>> 
>>>>>> This temporary variable is then used to access `domain_id` and
>>>>>> `vcpu_id`.
>>>>>> This ensures that the arguments passed to the `TRACE_TIME` macro 
>>>>>> are
>>>>>> simple expressions free of persistent side effects.
>>>>>> 
>>>>>> Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>
>>>>> The macro `current` specifically does not (and must not) have side
>>>>> effects.  It is expected to behave like a plain `struct vcpu 
>>>>> *current;`
>>>>> variable, and what Eclair is noticing is the thread-local machinery
>>>>> under this_cpu() (or in x86's case, get_current()).
>>>>> 
>>>>> In ARM's case, it's literally reading the hardware thread pointer
>>>>> register.  Can anything be done to tell Eclair that `this_cpu()`
>>>>> specifically does not have side effects?
>>>>> 
>>>>> The only reason that GVA_INFO() and TRACE_TIME() are picked out is
>>>>> because they both contain embedded structure initialisation, and
>>>>> this is
>>>>> is actually an example where trying to comply with MISRA interferes
>>>>> with
>>>>> what is otherwise a standard pattern in Xen.
>>>> Irrespective of what you say, some of the changes here were 
>>>> eliminating
>>>> multiple adjacent uses of current, which - iirc - often the compiler
>>>> can't fold via CSE.
>>> 
>>> Where we have mixed usage, sure.  (I'm sure I've got a branch 
>>> somewhere
>>> trying to add some more pure/const around to try and help out here, 
>>> but
>>> I can't find it, and don't recall it being a major improvement 
>>> either.)
>>> 
>>> The real problem here is that there are a *very few* number of 
>>> contexts
>>> where Eclair refuses to tolerate the use of `current` citing side
>>> effects, despite there being no side effects.
>>> 
>>> That is the thing that breaks the principle of least surprise, and we
>>> ought to fix it by making Eclair happy with `current` everywhere, 
>>> rather
>>> than force people to learn that 2 macros can't have a `current` in 
>>> their
>>> parameter list.
>>> 
>> 
>> I'll take a look. Likely yes, by adding a handful of properties. There
>> are subtleties, though.
>> 
> 
> Hi, Nicola.
> 
> Did you have a chance to try configure Eclair to ignore this macro
> `this_cpu()`?
> 

Hi Dmytro,

I'm on it, I needed to handle other tasks first.

> Thanks.
> Dmytro

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253