On Thu, Nov 1, 2018 at 8:56 AM, Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> wrote: > So if I have matching host kernels it should always work? > What happens if I upgrade the source kernel to increase it's maximum > nested size, can I force it to keep things small for some VMs? Any change to the format of the nested state should be gated by a KVM_CAP set by userspace. (Unlike, say, how the KVM_VCPUEVENT_VALID_SMM flag was added to the saved VCPU events state in commit f077825a8758d.) KVM has traditionally been quite bad about maintaining backwards compatibility, but I hope the community is more cognizant of the issues now. As a cloud provider, one would only enable the new capability from userspace once all hosts in the pool have a kernel that supports it. During the transition, the capability would not be enabled on the hosts with a new kernel, and these hosts would continue to provide nested state that could be consumed by hosts running the older kernel.
