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

[Jan Beulich <jbeulich@xxxxxxxx>]

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@xxxxxxxx>
> 
> 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.

Jan