Re: [PATCH v2 6/7] KVM: nVMX: Request to sync eVMCS from VMCS12 after migration

Maxim Levitsky
2021-05-24T12:35:46Z

On Mon, 2021-05-17 at 15:50 +0200, Vitaly Kuznetsov wrote:
> VMCS12 is used to keep the authoritative state during nested state
> migration. In case 'need_vmcs12_to_shadow_sync' flag is set, we're
> in between L2->L1 vmexit and L1 guest run when actual sync to
> enlightened (or shadow) VMCS happens. Nested state, however, has
> no flag for 'need_vmcs12_to_shadow_sync' so vmx_set_nested_state()->
> set_current_vmptr() always sets it. Enlightened vmptrld path, however,
> doesn't have the quirk so some VMCS12 changes may not get properly
> reflected to eVMCS and L1 will see an incorrect state.
>
> Note, during L2 execution or when need_vmcs12_to_shadow_sync is not
> set the change is effectively a nop: in the former case all changes
> will get reflected during the first L2->L1 vmexit and in the later
> case VMCS12 and eVMCS are already in sync (thanks to
> copy_enlightened_to_vmcs12() in vmx_get_nested_state()).
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/kvm/vmx/nested.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 3bfbf991bf45..a0dedd413a23 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3127,6 +3127,12 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
> if (evmptrld_status == EVMPTRLD_VMFAIL ||
> evmptrld_status == EVMPTRLD_ERROR)
> return false;
> +
> + /*
> + * Post migration VMCS12 always provides the most actual
> + * information, copy it to eVMCS upon entry.
> + */
> + vmx->nested.need_vmcs12_to_shadow_sync = true;

I noticed that bug too while reviewing the previous patches,
wasted some time thinking about it :-(

This does look like a clean and correct way to fix this, unless
you adopt my suggestion to sync evmcs right away on nested vmexit
as I explained in my review of patch 3 of this series.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky

> }
>
> return true;