On 01/02/2018 14:10, Radim Krčmář wrote: >>> >> This check will break migration if the source guest and host both have >> the recent kernels which support KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT, so >> I am not sure about it. Otherwise, the patch is okay! > Good point, breaking forward migration is worse than doing nothing. > > A compromise solution would be to drop the feature check from the > hypervisor. Newer guests would work everywhere and there would be no > change to old systems, so v4.13-v4.15 guests could at least upgrade. > > Slightly better than doing nothing, IMO, Yes, I agree. Just removing the guest_kvm_cpuid_has should be fine. Paolo