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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Nov 02, 2018 at 09:44:54AM -0700, Jim Mattson via Qemu-devel wrote:
> On Fri, Nov 2, 2018 at 5:59 AM, Liran Alon <liran.alon@oracle.com> wrote:
> >
> 
> >>> Therefore, I don't think that we want this versioning to be based on KVM_CAP at all.
> >>> It seems that we would want the process to behave as follows:
> >>> 1) Mgmt-layer at dest queries dest host max supported nested_state size.
> >>>   (Which should be returned from kvm_check_extension(KVM_CAP_NESTED_STATE))
> >>> 2) Mgmt-layer at source initiate migration to dest with requesting QEMU to send nested_state
> >>>   matching dest max supported nested_state size.
> >>>   When saving nested state using KVM_GET_NESTED_STATE IOCTL, QEMU will specify in nested_state->size
> >>>   the *requested* size to be saved and KVM should be able to save only the information which matches
> >>>   the version that worked with that size.
> >>> 3) After some sanity checks on received migration stream, dest host use KVM_SET_NESTED_STATE IOCTL.
> >>>   This IOCTL should deduce which information it should deploy based on given nested_state->size.
> 
> I have to object to any proposal which requires the management later
> to communicate with the source and the destination to determine what
> should be done.

Can you elaborate on why you object ?

There are a bunch of features in QEMU's migration code which require
the mgmt layer to look at source + dest to determine what should be
done. Admittedly the cases we have had so far are generic migration
features (compression, multifd, postcopy, TLS, etc), while this is
a host kernel feature. I don't think it is that far outside the
normal practice wrt migration feature usage decision making though.

Regards,
Daniel
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