On Fri, Jul 06, 2018 at 11:31:38AM +0200, Paolo Bonzini wrote: > On 06/07/2018 10:43, Daniel P. Berrangé wrote: > >> Based on the Linux patch at [1], guests may try to detect the > >> feature by writing to the MSR unconditionally. > >> > >> If this happens, KVM needs to provide a mechanism to > >> enable/disable the MSR emulation. Otherwise users will end up > >> with VMs that can't be migrated to older hosts even if they are > >> using older machine-types. > > Is there really no CPUID flag that can be used to detect the feature ? > > Unconditionally probing for existance of arbitrary MSRs seems to be > > just re-inventing CPUID feature detection, but worse because as you > > say we need to now invent a way to control existance of individual > > MSRs too :-( > > Now that I know that no silicon exists for this feature, the solution is > simple. The feature will be rejected until a CPUID bit exists.
This sounds even better than requiring a new mechanism to enable/disable MSRs in KVM. -- Eduardo
