Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES on nested vmexit

[Maxim Levitsky]

On Thu, 2021-01-07 at 18:51 +0100, Paolo Bonzini wrote:
> On 07/01/21 18:00, Sean Christopherson wrote:
> > Ugh, I assume this is due to one of the "premature" nested_ops->check_events()
> > calls that are necessitated by the event mess?  I'm guessing kvm_vcpu_running()
> > is the culprit?
> > 
> > If my assumption is correct, this bug affects nVMX as well.
> 
> Yes, though it may be latent.  For SVM it was until we started 
> allocating svm->nested on demand.
> 
> > Rather than clear the request blindly on any nested VM-Exit, what
> > about something like the following?
> 
> I think your patch is overkill, KVM_REQ_GET_NESTED_STATE_PAGES is only 
> set from KVM_SET_NESTED_STATE so it cannot happen while the VM runs.

Note that I didn't include the same fix for VMX becasue it uses a separate vmcs
for guest which has its own msr bitmap so in theory this shouldn't be needed,
but it won't hurt.

I'll test indeed if canceling the KVM_REQ_GET_NESTED_STATE_PAGES on VMX
makes any difference on VMX in regard to nested migration crashes I am seeing.

Best regards,
	Maxim Levitsky

> 
> Something like this is small enough and works well.
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index a622e63739b4..cb4c6ee10029 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -595,6 +596,8 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
>   	svm->nested.vmcb12_gpa = 0;
>   	WARN_ON_ONCE(svm->nested.nested_run_pending);
> 
> +	kvm_clear_request(KVM_REQ_GET_NESTED_STATE_PAGES, &svm->vcpu);
> +
>   	/* in case we halted in L2 */
>   	svm->vcpu.arch.mp_state = KVM_MP_STATE_RUNNABLE;
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index e2f26564a12d..0fbb46990dfc 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4442,6 +4442,8 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 
> vm_exit_reason,
>   	/* trying to cancel vmlaunch/vmresume is a bug */
>   	WARN_ON_ONCE(vmx->nested.nested_run_pending);
> 
> +	kvm_clear_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
> +
>   	/* Service the TLB flush request for L2 before switching to L1. */
>   	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
>   		kvm_vcpu_flush_tlb_current(vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3f7c1fc7a3ce..b7e784b5489c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8789,7 +8789,9 @@ 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 (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
> +			if (WARN_ON_ONCE(!is_guest_mode(&svm->vcpu)))
> +				;
> +			else if 
> (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
>   				r = 0;
>   				goto out;
>   			}
>