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

[Julien Grall <julien@xxxxxxx>]

Hi Nicola,

On 12/10/2023 16:28, 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.
In the commit message it is clear that the macro will return the lowest 
set bit. But...
> 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
> +
>   ### 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))
... this is not reflected in the name of the macro. So it is not obvious 
if it will return the lowest bit set or clear.
Can you at least add a comment on top explaining what it returns? 
Something like:
/* Return the lowest bit set */

Cheers,

--
Julien Grall