Re: [PATCH] x86/cpuid: prevent decreasing of hypervisor max leaf on migration

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Roger Pau Monne writes ("[PATCH] x86/cpuid: prevent decreasing of hypervisor 
max leaf on migration"):
> In order to be compatible with previous Xen versions, and not change
> max hypervisor leaf as a result of a migration, keep the clamping of
> the maximum leaf value provided to XEN_CPUID_MAX_NUM_LEAVES, instead
> of doing it based on the domain type. Also set the default maximum
> leaf without taking the domain type into account. The maximum
> hypervisor leaf is not migrated, so we need the default to not regress
> beyond what might already be reported to a guest by existing Xen
> versions.
> 
> This is a partial revert of 540d911c28 and restores the previous
> behaviour and assures that HVM guests won't see it's maximum
> hypervisor leaf reduced from 5 to 4 as a result of a migration.
> 
> Fixes: 540d911c28 ('x86/CPUID: shrink max_{,sub}leaf fields according to 
> actual leaf contents')
> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Cc: Ian Jackson <iwj@xxxxxxxxxxxxxx>
> ---
> Regarding release risks:
> 
> This is a partial revert of a commit.  The main issues could be that a
> partial revert could break the build or leave the remaining code in a
> non-working condition.
> 
> Breaking the build will be easily discovered by our automated testing,
> while leaving the remaining code in a broken state is unlikely, as the
> chunks reverted are isolated from the rest of the change in
> 540d911c28.

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

Thanks