Re: [PATCH v2 09/21] KVM: nVMX: Unconditionally clear mtf_pending on nested VM-Exit

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2022-06-14 at 20:47 +0000, Sean Christopherson wrote:
> Clear mtf_pending on nested VM-Exit instead of handling the clear on a
> case-by-case basis in vmx_check_nested_events().  The pending MTF should
> rever survive nested VM-Exit, as it is a property of KVM's run of the
^^ typo: never

Also it is not clear what the 'case by case' means.

I see that the vmx_check_nested_events always clears it unless nested run is pending
or we re-inject an event.



> current L2, i.e. should never affect the next L2 run by L1.  In practice,
> this is likely a nop as getting to L1 with nested_run_pending is
> impossible, and KVM doesn't correctly handle morphing a pending exception
> that occurs on a prior injected exception (need for re-injected exception
> being the other case where MTF isn't cleared).  However, KVM will
> hopefully soon correctly deal with a pending exception on top of an
> injected exception.



> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index d080bfca16ef..7b644513c82b 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3909,16 +3909,8 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>  	unsigned long exit_qual;
>  	bool block_nested_events =
>  	    vmx->nested.nested_run_pending || kvm_event_needs_reinjection(vcpu);
> -	bool mtf_pending = vmx->nested.mtf_pending;
>  	struct kvm_lapic *apic = vcpu->arch.apic;
>  
> -	/*
> -	 * Clear the MTF state. If a higher priority VM-exit is delivered first,
> -	 * this state is discarded.
> -	 */
> -	if (!block_nested_events)
> -		vmx->nested.mtf_pending = false;
> -
>  	if (lapic_in_kernel(vcpu) &&
>  		test_bit(KVM_APIC_INIT, &apic->pending_events)) {
>  		if (block_nested_events)
> @@ -3927,6 +3919,9 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>  		clear_bit(KVM_APIC_INIT, &apic->pending_events);
>  		if (vcpu->arch.mp_state != KVM_MP_STATE_INIT_RECEIVED)
>  			nested_vmx_vmexit(vcpu, EXIT_REASON_INIT_SIGNAL, 0, 0);
> +
> +		/* MTF is discarded if the vCPU is in WFS. */
> +		vmx->nested.mtf_pending = false;
>  		return 0;

I guess MTF should also be discarded if we enter SMM, and I see that
VMX also enter SMM with a pseudo VM exit (in vmx_enter_smm) which
will clear the MTF. Good.

>  	}
>  
> @@ -3964,7 +3959,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>  		return 0;
>  	}
>  
> -	if (mtf_pending) {
> +	if (vmx->nested.mtf_pending) {
>  		if (block_nested_events)
>  			return -EBUSY;
>  		nested_vmx_update_pending_dbg(vcpu);
> @@ -4562,6 +4557,9 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>  
> +	/* Pending MTF traps are discarded on VM-Exit. */
> +	vmx->nested.mtf_pending = false;
> +
>  	/* trying to cancel vmlaunch/vmresume is a bug */
>  	WARN_ON_ONCE(vmx->nested.nested_run_pending);
>  


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

Best regards,
	Maxim Levitsky