Re: [Qemu-devel] [PATCH] i386/kvm: Do not sync nested state during runtime

[Paolo Bonzini]

On 22/07/19 12:43, Jan Kiszka wrote:
>> Currently QEMU writes to userspace maintained nested-state only at 
>> kvm_arch_init_vcpu() and
>> when loading vmstate_nested_state vmstate subsection.
>> kvm_arch_reset_vcpu() do not modify userspace maintained nested-state.
> Hmm, then we probably achieve that effect by clearing the related bit in CR4.

Almost: by clearing the VMX enable bit in MSR_IA32_FEATURE_CONTROL.
Actually I think you contributed that. :)

I think we could in principle skip that MSR write if env->nested_state
!= NULL, but it doesn't hurt either, and it makes sense that nested virt
state goes together with MSR_IA32_FEATURE_CONTROL since the latter is
indede nested virtualization related.

Paolo

> If doing that implies an invalidation of the nested state by KVM, we
> are fine. Otherwise, I would expect userspace to reset the state to
> VMCLEAR and purge any traces of prior use.