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

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Roger Pau Monne writes ("[PATCH for-4.16] x86/cpuid: prevent shrinking migrated 
policies max leaves"):
> CPUID policies from guest being migrated shouldn't have the maximum
> leaves shrink, as that would be a guest visible change. The hypervisor
> has no knowledge on whether a guest has been migrated or is build from
> scratch, and hence it must not blindly shrink the CPUID policy in
> recalculate_cpuid_policy. Remove the
> x86_cpuid_policy_shrink_max_leaves call from recalculate_cpuid_policy.
> Removing such call could be seen as a partial revert of 540d911c28.
...
> This is a regression introduced in this release cycle, so we should
> consider whether we want to take this patch. It's mostly moving a
> shrink call from the hypervisor into the toolstack and making it more
> selective.
> 
> Main risks would be this shrinking somehow altering the recalculations
> of the CPUID policy done by the hypervisor. Removing the shirk itself
> in the hypervisor shouldn't cause issues as that wasn't done before,
> and reporting empty max leaf should be fine.

Thank you.

Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>