Re: [PATCH] x86/cpuid: do not expand max leaves on restore

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.04.2021 12:54, Roger Pau Monne wrote:
> When restoring limit the maximum leaves to the ones supported by Xen
> 4.13 in order to not expand the maximum leaf a guests sees. Note this
> is unlikely to cause real issues.

Why 4.13 (and not 4.12) here when ...

> Guests restored from Xen versions 4.13 or greater will contain CPUID
> data on the stream that will override the values set by
> xc_cpuid_apply_policy.

... 4.13 already communicates the values?

> --- a/tools/libs/guest/xg_cpuid_x86.c
> +++ b/tools/libs/guest/xg_cpuid_x86.c
> @@ -510,6 +510,11 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
> domid, bool restore,
>          {
>              p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
>          }
> +
> +        /* Clamp maximum leaves to the supported ones on 4.13. */

Same aspect here then. (While not being a native speaker, it
would still seem to me that flipping "supported" and "ones"
would read slightly more clearly.)

> +        p->basic.max_leaf = min(p->basic.max_leaf, 0xdu);
> +        p->feat.max_subleaf = min(p->feat.max_subleaf, 1u);
> +        p->extd.max_leaf = min(p->extd.max_leaf, 0x1cu);
>      }

With this I think the comment ahead of the enclosing if() wants
either amending or moving immediately inside the if()'s body.

Jan