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. Paolo
