KVM: x86: Add a fastpath handler for INVD

Add a fastpath handler for INVD so that the common fastpath logic can be
trivially tested on both Intel and AMD.  Under KVM, INVD is always:
(a) intercepted, (b) available to the guest, and (c) emulated as a nop,
with no side effects.  Combined with INVD not having any inputs or outputs,
i.e. no register constraints, INVD is the perfect instruction for
exercising KVM's fastpath as it can be inserted into practically any
guest-side code stream.

Link: https://lore.kernel.org/r/20250805190526.1453366-19-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -4200,6 +4200,8 @@ static fastpath_t svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 		return handle_fastpath_set_msr_irqoff(vcpu);
 	case SVM_EXIT_HLT:
 		return handle_fastpath_hlt(vcpu);
+	case SVM_EXIT_INVD:
+		return handle_fastpath_invd(vcpu);
 	default:
 		break;
 	}

arch/x86/kvm/vmx/vmx.c

@@ -7175,6 +7175,8 @@ static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu,
 		return handle_fastpath_preemption_timer(vcpu, force_immediate_exit);
 	case EXIT_REASON_HLT:
 		return handle_fastpath_hlt(vcpu);
+	case EXIT_REASON_INVD:
+		return handle_fastpath_invd(vcpu);
 	default:
 		return EXIT_FASTPATH_NONE;
 	}

arch/x86/kvm/x86.c

@@ -2087,6 +2087,15 @@ int kvm_emulate_invd(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_invd);
 
+fastpath_t handle_fastpath_invd(struct kvm_vcpu *vcpu)
+{
+	if (!kvm_emulate_invd(vcpu))
+		return EXIT_FASTPATH_EXIT_USERSPACE;
+
+	return EXIT_FASTPATH_REENTER_GUEST;
+}
+EXPORT_SYMBOL_GPL(handle_fastpath_invd);
+
 int kvm_handle_invalid_op(struct kvm_vcpu *vcpu)
 {
 	kvm_queue_exception(vcpu, UD_VECTOR);

arch/x86/kvm/x86.h

@@ -439,6 +439,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 			    int emulation_type, void *insn, int insn_len);
 fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu);
 fastpath_t handle_fastpath_hlt(struct kvm_vcpu *vcpu);
+fastpath_t handle_fastpath_invd(struct kvm_vcpu *vcpu);
 
 extern struct kvm_caps kvm_caps;
 extern struct kvm_host_values kvm_host;