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

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

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@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.
> 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