KVM: nVMX: Use vmcs01's controls shadow to check for IRQ/NMI windows at VM-Enter

Use vmcs01's execution controls shadow to check for IRQ/NMI windows after
a successful nested VM-Enter, instead of snapshotting the information prior
to emulating VM-Enter.  It's quite difficult to see that the entire reason
controls are snapshot prior nested VM-Enter is to read them from vmcs01
(vmcs02 is loaded if nested VM-Enter is successful).

That could be solved with a comment, but explicitly using vmcs01's shadow
makes the code self-documenting to a certain extent.

No functional change intended (vmcs01's execution controls must not be
modified during emulation of nested VM-Enter).

Link: https://lore.kernel.org/r/20241101191447.1807602-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/nested.c

@@ -3500,7 +3500,6 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
 	enum vm_entry_failure_code entry_failure_code;
-	bool evaluate_pending_interrupts;
 	union vmx_exit_reason exit_reason = {
 		.basic = EXIT_REASON_INVALID_STATE,
 		.failed_vmentry = 1,
@@ -3519,9 +3518,6 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 
 	kvm_service_local_tlb_flush_requests(vcpu);
 
-	evaluate_pending_interrupts = exec_controls_get(vmx) &
-		(CPU_BASED_INTR_WINDOW_EXITING | CPU_BASED_NMI_WINDOW_EXITING);
-
 	if (!vmx->nested.nested_run_pending ||
 	    !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS))
 		vmx->nested.pre_vmenter_debugctl = vmcs_read64(GUEST_IA32_DEBUGCTL);
@@ -3604,9 +3600,11 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 	 * Re-evaluate pending events if L1 had a pending IRQ/NMI/INIT/SIPI
 	 * when it executed VMLAUNCH/VMRESUME, as entering non-root mode can
 	 * effectively unblock various events, e.g. INIT/SIPI cause VM-Exit
-	 * unconditionally.
+	 * unconditionally.  Take care to pull data from vmcs01 as appropriate,
+	 * e.g. when checking for interrupt windows, as vmcs02 is now loaded.
 	 */
-	if (unlikely(evaluate_pending_interrupts) ||
+	if ((__exec_controls_get(&vmx->vmcs01) & (CPU_BASED_INTR_WINDOW_EXITING |
+						  CPU_BASED_NMI_WINDOW_EXITING)) ||
 	    kvm_apic_has_pending_init_or_sipi(vcpu) ||
 	    kvm_apic_has_interrupt(vcpu))
 		kvm_make_request(KVM_REQ_EVENT, vcpu);