Re: [Qemu-devel] [QEMU PATCH v2 0/2]: KVM: i386: Add support for save and restore nested state

[Jim Mattson]

On Thu, Nov 1, 2018 at 8:56 AM, Dr. David Alan Gilbert
<address@hidden> 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.