Re: [XEN PATCH][for-next][for-4.19 v2 1/8] xen/include: add macro LOWEST_BIT

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 12 Oct 2023, Nicola Vetrini wrote:
> The purpose of this macro is to encapsulate the well-known expression
> 'x & -x', that in 2's complement architectures on unsigned integers will
> give 2^ffs(x), where ffs(x) is the position of the lowest set bit in x.
> 
> A deviation for ECLAIR is also introduced.
> 
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> ---
> Changes in v2:
> - rename to LOWEST_BIT
> ---
>  automation/eclair_analysis/ECLAIR/deviations.ecl | 6 ++++++
>  xen/include/xen/macros.h                         | 6 ++++--
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/automation/eclair_analysis/ECLAIR/deviations.ecl 
> b/automation/eclair_analysis/ECLAIR/deviations.ecl
> index d8170106b449..b8e1155ee49d 100644
> --- a/automation/eclair_analysis/ECLAIR/deviations.ecl
> +++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
> @@ -274,6 +274,12 @@ still non-negative."
>  -config=MC3R1.R10.1,etypes+={safe, 
> "stmt(operator(logical)||node(conditional_operator||binary_conditional_operator))",
>  "dst_type(ebool||boolean)"}
>  -doc_end
> 
> +-doc_begin="The macro LOWEST_BIT encapsulates a well-known pattern to obtain 
> the value
> +2^ffs(x) for unsigned integers on two's complement architectures
> +(all the architectures supported by Xen satisfy this requirement)."
> +-config=MC3R1.R10.1,reports+={safe, 
> "any_area(any_loc(any_exp(macro(^LOWEST_BIT$))))"}
> +-doc_end

Please also add the same deviation and explanation to
docs/misra/deviations.rst. Other than that, this looks fine.


>  ### Set 3 ###
> 
>  #
> diff --git a/xen/include/xen/macros.h b/xen/include/xen/macros.h
> index d0caae7db298..af47179d1056 100644
> --- a/xen/include/xen/macros.h
> +++ b/xen/include/xen/macros.h
> @@ -8,8 +8,10 @@
>  #define DIV_ROUND(n, d) (((n) + (d) / 2) / (d))
>  #define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
> 
> -#define MASK_EXTR(v, m) (((v) & (m)) / ((m) & -(m)))
> -#define MASK_INSR(v, m) (((v) * ((m) & -(m))) & (m))
> +#define LOWEST_BIT(x) ((x) & -(x))
> +
> +#define MASK_EXTR(v, m) (((v) & (m)) / LOWEST_BIT(m))
> +#define MASK_INSR(v, m) (((v) * LOWEST_BIT(m)) & (m))
> 
>  #define count_args_(dot, a1, a2, a3, a4, a5, a6, a7, a8, x, ...) x
>  #define count_args(args...) \
> --
> 2.34.1
>