Re: [PATCH 7/9] x86/hvm: Disable MPX by default

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.06.2020 16:15, Andrew Cooper wrote:
> @@ -479,6 +497,18 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
> domid, bool restore,
>          goto out;
>      }
>  
> +    /*
> +     * Account for feature which have been disabled by default since Xen 
> 4.13,
> +     * so migrated-in VM's don't risk seeing features disappearing.
> +     */
> +    if ( restore )
> +    {
> +        if ( di.hvm )
> +        {
> +            p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);

Why do you derive this from the host featureset instead of the max
one for the guest type? Also, while you modify p here, ...

> +        }
> +    }
> +
>      if ( featureset )
>      {
>          uint32_t disabled_features[FEATURESET_NR_ENTRIES],

... the code in this if()'s body ignores p altogether. I realize the
only caller of the function passes NULL for "featureset", but I'd
like to understand the rationale here anyway before giving an R-b.

Jan