Re: [PATCH for-4.16] x86/cpuid: prevent shrinking migrated policies max leaves

[Andrew Cooper <amc96@xxxxxxxx>]

On 10/11/2021 17:40, Roger Pau Monne wrote:
> diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
> index 198892ebdf..3ffd5f683b 100644
> --- a/tools/libs/guest/xg_cpuid_x86.c
> +++ b/tools/libs/guest/xg_cpuid_x86.c
> @@ -638,6 +638,13 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
> domid, bool restore,
>          }
>      }
>  
> +    /*
> +     * Do not try to shrink the policy if restoring, as that could cause
> +     * guest visible changes in the maximum leaf fields.
> +     */
> +    if ( !restore )
> +        x86_cpuid_policy_shrink_max_leaves(p);

Nothing in xc_cpuid_apply_policy() changes any of the max leaves, so
this is dead logic.

xc_cpuid_xend_policy() can in principle change max leaves, but that
logic is all horribly broken and I don't recommend touching it.

I'd just drop this hunk entirely, and retain the deletion in the hypervisor.

~Andrew