Liran Alon <liran.alon@xxxxxxxxxx> writes: >>Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes: > >>>Liran Alon <liran.alon@xxxxxxxxxx> writes: > >>> It seems to me from what you describe that Hyper-V has a bug of using revision_id specified in IA32_VMX_BASIC_MSR >>> instead of using one of the eVMCS supported versions reported in CPUID.4000000A. >>> >>> I’m also not sure I understand what you suggest as “change the other side of KVM to make it’s behaviour match Hyper-V”. >>> Do you mean that we will change KVM usage of eVMCS such that >>> alloc_vmcs_cpu() will always set vmcs->hdr.revision_id to >>> vmcs_config.revision_id? > >> Yes. My current understanding (correct me if I'm wrong!) is that both >> KVM-on-Hyper-V and Hyper-V-on-KVM work well with Enlightened >> VMCS. Surprisingly, KVM-on-KVM doesn't - and this is because of the >> issue you're trying to address by the patch. > > Yes you understand correctly. > > Well, I can change alloc_vmcs_cpu() to always set vmcs->hdr.revison_id to vmcs_config.revision_id even when eVMCS is enabled > (which is a deliberate wrong behaviour of KVM eVMCS client code) and also not apply this patch (which remains a deliberate wrong behaviour of KVM eVMCS server code). > All of this to overcome Hyper-V wrong behaviour :) Alternatively we can just drop the check in nested_vmx_handle_enlightened_vmptrld() for now: The only supported eVMCS version - both by Hyper-V and KVM is '1' and we explicitly advertise that in CPUID.4000000A so even when there's a new Hyper-V version supporting e.g. eVMCS2 it won't be used. Microsoft will also have to fix the issue when this happens - or how else would they know which eVMCS version is in use by the guest if they always copy IA32_VMX_BASIC_MSR? > > Paolo, do you think we should insert this wrong behaviour to KVM to support running on top of buggy Hyper-V? I unfortunately believe we should. > If so, should I insert this weird behaviour behind a kvm-intel module > parameter? E.g. compact_evmcs_version? Not Paolo but I'd say this is an overkill. And, even if we decide to go down this road, I think that the default KVM configuration should support real world Hyper-V so the module parameter should rather be 'correct_evmcs_versioning' :-) -- Vitaly
