Re: [XEN PATCH] xen/types: address Rule 10.1 for macro BITS_TO_LONGS

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 6 Sep 2023, Jan Beulich wrote:
> On 06.09.2023 17:57, Nicola Vetrini wrote:
> > On 05/09/2023 10:33, Jan Beulich wrote:
> >> On 05.09.2023 10:20, Nicola Vetrini wrote:
> >>> On 05/09/2023 09:46, Jan Beulich wrote:
> >>>> On 05.09.2023 09:31, Nicola Vetrini wrote:
> >>>>> Given its use in the declaration
> >>>>> 'DECLARE_BITMAP(features, IOMMU_FEAT_count)' the argument
> >>>>> 'bits' has essential type 'enum iommu_feature', which is not
> >>>>> allowed by the Rule as an operand to the addition operator.
> >>>>> Given that its value can be represented by a signed integer,
> >>>>> the explicit cast resolves the violation.
> >>>>
> >>>> Wait - why would this lead to a change to BITS_TO_LONGS()? And if 
> >>>> that
> >>>> was to be changed, why plain int? I don't think negative input makes
> >>>> sense there, and in principle I'd expect values beyond 4 billion to
> >>>> also be permissible (even if likely no such use will ever appear in a
> >>>> DECLARE_BITMAP(), but elsewhere it may make sense). Even going to
> >>>> "unsigned long" may be too limiting ...
> >>>>
> >>>
> >>> You have a point. I can think of doing it like this:
> >>> DECLARE_BITMAP(features, (int)IOMMU_FEAT_count)

I think this is a good solution for this case (even more so if we can't
find a better implementation of BITS_TO_LONGS)


> >>> on the grounds that the enum constant is representable in an int, and 
> >>> it
> >>> does not seem likely
> >>> to get much bigger.
> >>> Having an unsigned cast requires making the whole expression
> >>> essentially unsigned, otherwise Rule 10.4 is violated because
> >>> BITS_PER_LONG is
> >>> essentially signed. This can be done, but it depends on how
> >>> BITS_TO_LONGS will be/is used.
> >>
> >> It'll need looking closely, yes, but I expect that actually wants to be 
> >> an
> >> unsigned constant. I wouldn't be surprised if some use of 
> >> DECLARE_BITMAP()
> >> appeared (or already existed) where the 2nd argument involves sizeof() 
> >> in
> >> some way.
> >>
> > 
> > I think there's one with ARRAY_SIZE. In my opinion this can be resolved 
> > as follows:
> > 
> > #define BYTES_PER_LONG (1U << LONG_BYTEORDER) // the essential type gets 
> > from signed to unsigned
> > 
> > #define BITS_TO_LONGS(bits) \
> >          (((unsigned long long)(bits)+BITS_PER_LONG-1U)/BITS_PER_LONG) // 
> > same here
> 
> Except, as said before, I consider any kind of cast on "bits" latently
> problematic.

Can't we just do this (same but without the cast):

#define BYTES_PER_LONG (1U << LONG_BYTEORDER)
#define BITS_TO_LONGS(bits) \
         (((bits)+BITS_PER_LONG-1U)/BITS_PER_LONG)

Then we just need to make sure to pass an unsigned to BITS_TO_LONGS. In
the case above we would do:

DECLARE_BITMAP(features, (unsigned int)IOMMU_FEAT_count)