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

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 07/09/2023 03:33, Stefano Stabellini wrote:
> 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)

There is a build error due to -Werror because of a pointer comparison at 
line 469 of common/numa.c:
i = min(PADDR_BITS, BITS_PER_LONG - 1);
where
#define PADDR_BITS              52

I guess PADDR_BITS can become unsigned or gain a cast

-- 
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)