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

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

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.

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