Re: [XEN PATCH 4/6] x86emul: address violations of MISRA C Rule 20.7

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 2024-06-12 11:19, Jan Beulich wrote:
> On 11.06.2024 17:53, Nicola Vetrini wrote:
> > MISRA C Rule 20.7 states: "Expressions resulting from the expansion
> > of macro parameters shall be enclosed in parentheses". Therefore, some
> > macro definitions should gain additional parentheses to ensure that 
> > all
> > current and future users will be safe with respect to expansions that
> > can possibly alter the semantics of the passed-in macro parameter.
> >
> > No functional change.
> >
> > Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> > ---
> > These local helpers could in principle be deviated, but the 
> > readability
> > and functionality are essentially unchanged by complying with the 
> > rule,
> > so I decided to modify the macro definition as the preferred option.
> Well, yes, but ...
>
> > --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> > +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> > @@ -2255,7 +2255,7 @@ x86_emulate(
> >          switch ( modrm_reg & 7 )
> >          {
> >  #define GRP2(name, ext) \
> > -        case ext: \
> > +        case (ext): \
> >              if ( ops->rmw && dst.type == OP_MEM ) \
> >                  state->rmw = rmw_##name; \
> >              else \
> > @@ -8611,7 +8611,7 @@ int x86_emul_rmw(
> >              unsigned long dummy;
> >
> >  #define XADD(sz, cst, mod) \
> > -        case sz: \
> > +        case (sz): \
> >              asm ( "" \
> >                    COND_LOCK(xadd) " %"#mod"[reg], %[mem]; " \
> >                    _POST_EFLAGS("[efl]", "[msk]", "[tmp]") \
> ... this is really nitpicky of the rule / tool. What halfway realistic
> ways do you see to actually misuse these macros? What follows the 
> "case"
> keyword is just an expression; no precedence related issues are 
> possible.
I do share the view: no real danger is possible in sensible uses. Often 
MISRA rules are stricter than necessary to have a simple formulation, by 
avoiding too many special cases.
However, if a deviation is formulated, then it needs to be maintained, 
for no real readability benefit in this case, in my opinion. I can be 
convinced otherwise, of course.
--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)