Re: [XEN PATCH 5/9] x86/cpu-policy: address violations of MISRA C Rule 10.1

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/10/2023 9:26 am, Nicola Vetrini wrote:
> The COUNT_LEAVES macro is introduced to avoid using an essentially
> boolean value in a subtraction.
>
> No functional change.
>
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> ---
>  xen/include/xen/lib/x86/cpu-policy.h | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/xen/include/xen/lib/x86/cpu-policy.h 
> b/xen/include/xen/lib/x86/cpu-policy.h
> index bab3eecda6c1..700993cc67e8 100644
> --- a/xen/include/xen/lib/x86/cpu-policy.h
> +++ b/xen/include/xen/lib/x86/cpu-policy.h
> @@ -95,17 +95,18 @@ const char *x86_cpuid_vendor_to_str(unsigned int vendor);
>  #define CPUID_GUEST_NR_EXTD       MAX(CPUID_GUEST_NR_EXTD_INTEL, \
>                                        CPUID_GUEST_NR_EXTD_AMD)
>  
> +#define COUNT_LEAVES(X) ((X) - ((X) ? 1 : 0))
>  /*
>   * Maximum number of leaves a struct cpu_policy turns into when serialised 
> for
>   * interaction with the toolstack.  (Sum of all leaves in each union, less 
> the
>   * entries in basic which sub-unions hang off of.)
>   */
> -#define CPUID_MAX_SERIALISED_LEAVES                     \
> -    (CPUID_GUEST_NR_BASIC +                             \
> -     CPUID_GUEST_NR_FEAT   - !!CPUID_GUEST_NR_FEAT +    \
> -     CPUID_GUEST_NR_CACHE  - !!CPUID_GUEST_NR_CACHE +   \
> -     CPUID_GUEST_NR_TOPO   - !!CPUID_GUEST_NR_TOPO +    \
> -     CPUID_GUEST_NR_XSTATE - !!CPUID_GUEST_NR_XSTATE +  \
> +#define CPUID_MAX_SERIALISED_LEAVES         \
> +    (CPUID_GUEST_NR_BASIC +                 \
> +     COUNT_LEAVES(CPUID_GUEST_NR_FEAT) +    \
> +     COUNT_LEAVES(CPUID_GUEST_NR_CACHE) +   \
> +     COUNT_LEAVES(CPUID_GUEST_NR_TOPO) +    \
> +     COUNT_LEAVES(CPUID_GUEST_NR_XSTATE) +  \
>       CPUID_GUEST_NR_EXTD + 2 /* hv_limit and hv2_limit */ )

This may not have been a MISRA-approved calculation, but encapsulating
it like this breaks any ability to follow what's going on.

CPUID data in x86 is mostly a sparse 1-D array (BASIC, EXTD, HV blocks),
but a couple of elements in the BASIC array have arrays themselves.

The struct is laid out for O(1) access, so you can't just say
sizeof(struct) / sizeof(element).  The BASIC array has elements (0x4,
0x7, 0xb, 0xd) which hold no data because there's actually an array
elsewhere containing all the data.

So logically, it's:

(BASIC + (FEAT - 1) + (CACHE - 1) + (TOPO - 1) + (XSTATE - 1)) + EXTD + 2

And in practice I'd far rather express it with a plain -1 than a -
!!NR_, if the latter isn't an option.

Presumably MISRA would be happy with that?

If so, I can submit a patch.  There's also a typo in that the comment
that wants fixing.

~Andrew