Re: [PATCH 11/21] libs/guest: allow updating a cpu policy CPUID data

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.03.2021 10:58, Roger Pau Monne wrote:
> --- a/tools/libs/guest/xg_cpuid_x86.c
> +++ b/tools/libs/guest/xg_cpuid_x86.c
> @@ -966,3 +966,70 @@ int xc_cpu_policy_get_msr(xc_interface *xch, const 
> xc_cpu_policy_t policy,
>      free(msrs);
>      return rc;
>  }
> +
> +int xc_cpu_policy_update_cpuid(xc_interface *xch, xc_cpu_policy_t policy,
> +                               const xen_cpuid_leaf_t *leaves,
> +                               uint32_t nr)
> +{
> +    unsigned int err_leaf = -1, err_subleaf = -1;
> +    unsigned int nr_leaves, nr_msrs, i, j;
> +    xen_cpuid_leaf_t *current;
> +    int rc = xc_cpu_policy_get_size(xch, &nr_leaves, &nr_msrs);
> +
> +    if ( rc )
> +    {
> +        PERROR("Failed to obtain policy info size");
> +        return -1;
> +    }
> +
> +    current = calloc(nr_leaves, sizeof(*current));
> +    if ( !current )
> +    {
> +        PERROR("Failed to allocate resources");
> +        errno = ENOMEM;
> +        return -1;
> +    }
> +
> +    rc = xc_cpu_policy_serialise(xch, policy, current, &nr_leaves, NULL, 0);
> +    if ( rc )
> +        goto out;
> +
> +    for ( i = 0; i < nr; i++ )
> +    {
> +        const xen_cpuid_leaf_t *update = &leaves[i];
> +
> +        for ( j = 0; j < nr_leaves; j++ )
> +            if ( current[j].leaf == update->leaf &&
> +                 current[j].subleaf == update->subleaf )
> +            {
> +                /*
> +                 * NB: cannot use an assignation because of the const vs
> +                 * non-const difference.
> +                 */
> +                memcpy(&current[j], update, sizeof(*update));

I'm having trouble understanding the comment. In

    current[j] = *update;

the lvalue is xen_cpuid_leaf_t and the rvalue is const xen_cpuid_leaf_t.
That the usual (and permitted) arrangement afaics.

Jan