KVM: nVMX: Fix pending events injection

L2 fails to boot on a non-APICv box dues to 'commit 0ad3bed
("kvm: nVMX: move nested events check to kvm_vcpu_running")'

KVM internal error. Suberror: 3
extra data[0]: 800000ef
extra data[1]: 1
RAX=0000000000000000 RBX=ffffffff81f36140 RCX=0000000000000000 RDX=0000000000000000
RSI=0000000000000000 RDI=0000000000000000 RBP=ffff88007c92fe90 RSP=ffff88007c92fe90
R8 =ffff88007fccdca0 R9 =0000000000000000 R10=00000000fffedb3d R11=0000000000000000
R12=0000000000000003 R13=0000000000000000 R14=0000000000000000 R15=ffff88007c92c000
RIP=ffffffff810645e6 RFL=00000246 [---Z-P-] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0000 0000000000000000 ffffffff 00c00000
CS =0010 0000000000000000 ffffffff 00a09b00 DPL=0 CS64 [-RA]
SS =0000 0000000000000000 ffffffff 00c00000
DS =0000 0000000000000000 ffffffff 00c00000
FS =0000 0000000000000000 ffffffff 00c00000
GS =0000 ffff88007fcc0000 ffffffff 00c00000
LDT=0000 0000000000000000 ffffffff 00c00000
TR =0040 ffff88007fcd4200 00002087 00008b00 DPL=0 TSS64-busy
GDT=     ffff88007fcc9000 0000007f
IDT=     ffffffffff578000 00000fff
CR0=80050033 CR2=00000000ffffffff CR3=0000000001e0a000 CR4=003406e0
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000
DR6=00000000fffe0ff0 DR7=0000000000000400
EFER=0000000000000d01

We should try to reinject previous events if any before trying to inject
new event if pending. If vmexit is triggered by L2 guest and L0 interested
in, we should reinject IDT-vectoring info to L2 through vmcs02 if any,
otherwise, we can consider new IRQs/NMIs which can be injected and call
nested events callback to switch from L2 to L1 if needed and inject the
proper vmexit events. However, 'commit 0ad3bed ("kvm: nVMX: move
nested events check to kvm_vcpu_running")' results in the handle events
order reversely on non-APICv box. This patch fixes it by bailing out for
pending events and not consider new events in this scenario.

Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Jan Kiszka <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Fixes: 0ad3bed ("kvm: nVMX: move nested events check to kvm_vcpu_running")
Signed-off-by: Radim Krčmář <[email protected]>

arch/x86/kvm/vmx.c

@@ -10641,6 +10641,11 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
+	if (vcpu->arch.exception.pending ||
+		vcpu->arch.nmi_injected ||
+		vcpu->arch.interrupt.pending)
+		return -EBUSY;
+
 	if (nested_cpu_has_preemption_timer(get_vmcs12(vcpu)) &&
 	    vmx->nested.preemption_timer_expired) {
 		if (vmx->nested.nested_run_pending)
@@ -10650,8 +10655,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
 	}
 
 	if (vcpu->arch.nmi_pending && nested_exit_on_nmi(vcpu)) {
-		if (vmx->nested.nested_run_pending ||
-		    vcpu->arch.interrupt.pending)
+		if (vmx->nested.nested_run_pending)
 			return -EBUSY;
 		nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI,
 				  NMI_VECTOR | INTR_TYPE_NMI_INTR |