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

[Roger Pau Monné]

On Tue, Mar 30, 2021 at 05:56:35PM +0200, Jan Beulich wrote:
> 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.

I'm sure I was doing something really stupid, and as a bonus I failed
to properly parse the error message I got from the compiler. It's now
fixed here and below.

Thanks, Roger.