Re: [XEN PATCH][for-4.19 8/9] xen/types: address Rule 10.1 for DECLARE_BITMAP use

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

> > > > I agree on everything Julien's wrote and I was about to suggest to 
> > > > use a
> > > > SAF comment to suppress the warning because it is clearer than a int
> > > > cast.
> > > >
> > > > But then I realized that even if BITS_TO_LONGS was fixed, wouldn't 
> > > > still
> > > > we have a problem because IOMMU_FEAT_count is an enum?
> > > >
> > > > Is it the case that IOMMU_FEAT_count would have to be cast 
> > > > regardless,
> > > > either to int or unsigned int or whatever to be used in 
> > > > DECLARE_BITMAP?
> > > >
> > > > So we have 2 problems here: one problem is DECLARE_BITMAP taking int
> > > > instead of unsigned int, and another problem is IOMMU_FEAT_count 
> > > > being
> > > > an enum.
> > > >
> > > > If I got it right, then I would go with the cast to int (like done 
> > > > in
> > > > this patch) with a decent comment on top of it.
> > > I might be missing something here. But why should we use cast rather 
> > > than /*
> > > SAF-X */ just above? I would have expected we wanted to highlight the
> > > violation rather than hiding it.
> > My understanding is that the cast is required when converting an enum
> > type to an integer type and vice versa. The idea is that we shouldn't 
> > do
> > implicit conversions as they are error prone, only explicit 
> > conversions
> > are allowed between enum and integers.
> We have a lot of places in Xen using implicit conversion from enum to
> integer (for instance in the P2M code for p2m_type_t). Does ECLAIR
> report all of them? If not, then why?
Can you give some pointers as to where this enum is used in arithmetic 
operations?
From a cursory glace I can see equality comparisons and
as arguments to the array subscript operator, which are both compliant.

> > So we need either (int) or (unsigned int). The question is which one
> > between the two, and theoretically (unsigned int) is better but due to
> > the reasons above (int) is the simplest choice.
> >
> > Yes, instead of the cast we can also add a SAF-x comment, which refers
> > to a deviation that says something along the lines "we could fix this
> > with a cast but we prefer a deviation because it makes the code easier
> > to read".
> >
> > In general my personal preference would be to use a cast, because we
> > shouldn't implicitly convert enums to integers.
> See above. I'd like to understand whether we are going to sprinkle the
> code with cast. If so, I am afraid I am against this solution.
>
> Cheers,
--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)