Re: [XEN PATCH][for-4.19 v3 1/8] xen/include: add macro LOWEST_BIT

[Jan Beulich <jbeulich@suse.com>]

On 20.10.2023 17:28, Nicola Vetrini wrote:
> --- a/automation/eclair_analysis/ECLAIR/deviations.ecl
> +++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
> @@ -246,6 +246,12 @@ constant expressions are required.\""
>    "any()"}
>  -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

This being deviated this way (rather than by SAF-* comments) wants
justifying in the description. You did reply to my respective
comment on v2, but such then (imo) needs propagating into the actual
patch as well.

> --- a/docs/misra/deviations.rst
> +++ b/docs/misra/deviations.rst
> @@ -192,6 +192,13 @@ Deviations related to MISRA C:2012 Rules:
>         See automation/eclair_analysis/deviations.ecl for the full explanation.
>       - Tagged as `safe` for ECLAIR.
>  
> +   * - R10.1
> +     - The well-known pattern (x & -x) applied to unsigned integer values on 2's
> +       complement architectures (i.e., all architectures supported by Xen), used
> +       to obtain the value 2^ffs(x), where ffs(x) is the position of the first
> +       bit set. If no bits are set, zero is returned.
> +     - Tagged as `safe` for ECLAIR.

In such an explanation there shall not be any ambiguity. Here I see
an issue with ffs() returning 1-based bit position numbers, which
isn't in line with the use in 2^ffs(x). (Arguably use of ^ itself is
also problematic, as that's XOR in C, not POW. I'd suggest 2^^ffs(x)
or 2**ffs(x).)

> --- a/xen/include/xen/macros.h
> +++ b/xen/include/xen/macros.h
> @@ -8,8 +8,11 @@
>  #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))
> +/* Returns the 2^ffs(x) or 0, where ffs(x) is the index of the lowest set bit */
> +#define LOWEST_BIT(x) ((x) & -(x))

I'm afraid my concern regarding this new macro's name (voiced on v2) hasn't
been addressed (neither verbally nor by finding a more suitable name).

Jan