Re: [PATCH] KVM: x86: allow KVM_REQ_GET_NESTED_STATE_PAGES outside guest mode for VMX

[Sean Christopherson]

On Mon, Jan 25, 2021, Paolo Bonzini wrote:
> +static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> +{
> +     struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +     struct vcpu_vmx *vmx = to_vmx(vcpu);
> +     struct kvm_host_map *map;
> +     struct page *page;
> +     u64 hpa;
> +
> +     if (!nested_get_evmcs_page(vcpu))
> +             return false;
>  
>       if (nested_cpu_has2(vmcs12, SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES)) {
>               /*
> @@ -3224,6 +3233,17 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu 
> *vcpu)
>       return true;
>  }
>  
> +static bool vmx_get_nested_state_pages(struct kvm_vcpu *vcpu)
> +{
> +     if (!nested_get_evmcs_page(vcpu))
> +             return false;
> +
> +     if (is_guest_mode(vcpu) && !nested_get_vmcs12_pages(vcpu))
> +             return false;

nested_get_evmcs_page() will get called twice in the common case of
is_guest_mode() == true.  I can't tell if that will ever be fatal, but it's
definitely weird.  Maybe this?

        if (!is_guest_mode(vcpu))
                return nested_get_evmcs_page(vcpu);

        return nested_get_vmcs12_pages(vcpu);

> +
> +     return true;
> +}
> +
>  static int nested_vmx_write_pml_buffer(struct kvm_vcpu *vcpu, gpa_t gpa)
>  {
>       struct vmcs12 *vmcs12;
> @@ -6602,7 +6622,7 @@ struct kvm_x86_nested_ops vmx_nested_ops = {
>       .hv_timer_pending = nested_vmx_preemption_timer_pending,
>       .get_state = vmx_get_nested_state,
>       .set_state = vmx_set_nested_state,
> -     .get_nested_state_pages = nested_get_vmcs12_pages,
> +     .get_nested_state_pages = vmx_get_nested_state_pages,
>       .write_log_dirty = nested_vmx_write_pml_buffer,
>       .enable_evmcs = nested_enable_evmcs,
>       .get_evmcs_version = nested_get_evmcs_version,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9a8969a6dd06..b910aa74ee05 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8802,9 +8802,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  
>       if (kvm_request_pending(vcpu)) {
>               if (kvm_check_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu)) {
> -                     if (WARN_ON_ONCE(!is_guest_mode(vcpu)))
> -                             ;
> -                     else if 
> (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
> +                     if 
> (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
>                               r = 0;
>                               goto out;
>                       }
> -- 
> 2.26.2
>