Re: [Qemu-devel] [PATCH 3/7] KVM: i386: Add support for KVM_CAP_EXCEPTION_PAYLOAD

[Paolo Bonzini]

On 17/06/19 13:34, Liran Alon wrote:
> Putting this all together, in case kernel doesn’t support extracting
> nested-state, there is no decent way to know if guest is running
> nested-virtualization. Which means that in theory we always need to
> fail migration in case kernel doesn’t support KVM_CAP_NESTED_STATE or
> KVM_CAP_EXCEPTION_PAYLOAD and vCPU is exposed with VMX/SVM
> capability.

For VMX this would be okay because we had a blocker before this series,
and this wouldn't be any worse.

For SVM we can ignore the case and fix it when we have
KVM_CAP_NESTED_STATE, as again that wouldn't be any worse.

Paolo

> I can condition this behaviour with a flag that can be manipulated
> using QMP to allow user to indicate it wishes to migrate guest anyway
> in this case. This however bring me back to the entire discussion I
> had with Dr. David Alan Gilbert on migration backwards compatibility
> in general and the fact that I believe we should have a generic QMP
> command which allows to provide list of VMState subsections that can
> be ignored in migration… See:
> https://www.mail-archive.com/address@hidden/msg622274.html
> 
> Paolo, What are your thoughts on how I would proceed with this?