KVM: x86: Extract VMX's unhandleable emulation check to common x86

Expose VMX's check for unhandleable emulation as its own kvm_x86_ops hook,
and move the actual pre-KVM_RUN check into common x86.  This will allow
sharing the core logic with KVM's RSM emulation without needed to add a
post-RSM hook, and is a step towards removing the .vcpu_pre_run() hook
entirely.

Alternatively, KVM could provide a post-RSM hook as mentioned, but pre/post
hooks tend to be unwieldy as the exact "timing" of the call often matters
greatly.  E.g. in this case, the call must slot in exactly between loading
guest state from SMRAM and the hack to force the vCPU out of L2 on SHUTDOWN.

Link: https://patch.msgid.link/20260731173340.2644656-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/kvm-x86-ops.h

@@ -68,6 +68,7 @@ KVM_X86_OP(vcpu_run)
 KVM_X86_OP(handle_exit)
 KVM_X86_OP(skip_emulated_instruction)
 KVM_X86_OP_OPTIONAL(update_emulated_instruction)
+KVM_X86_OP_OPTIONAL_RET0(unhandleable_emulation_required)
 KVM_X86_OP(set_interrupt_shadow)
 KVM_X86_OP(get_interrupt_shadow)
 KVM_X86_OP(patch_hypercall)

arch/x86/include/asm/kvm_host.h

@@ -1591,6 +1591,8 @@ struct kvm_x86_ops {
 		enum exit_fastpath_completion exit_fastpath);
 	int (*skip_emulated_instruction)(struct kvm_vcpu *vcpu);
 	void (*update_emulated_instruction)(struct kvm_vcpu *vcpu);
+	bool (*unhandleable_emulation_required)(struct kvm_vcpu *vcpu);
+
 	void (*set_interrupt_shadow)(struct kvm_vcpu *vcpu, int mask);
 	u32 (*get_interrupt_shadow)(struct kvm_vcpu *vcpu);
 	void (*patch_hypercall)(struct kvm_vcpu *vcpu,

arch/x86/kvm/vmx/main.c

@@ -165,6 +165,16 @@ static int vt_handle_exit(struct kvm_vcpu *vcpu,
 	return vmx_handle_exit(vcpu, fastpath);
 }
 
+static bool vt_unhandleable_emulation_required(struct kvm_vcpu *vcpu)
+{
+	if (is_td_vcpu(vcpu)) {
+		WARN_ON_ONCE(to_vt(vcpu)->emulation_required);
+		return false;
+	}
+
+	return vmx_unhandleable_emulation_required(vcpu);
+}
+
 static int vt_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 {
 	if (unlikely(is_td_vcpu(vcpu)))
@@ -944,6 +954,7 @@ struct kvm_x86_ops vt_x86_ops __initdata = {
 	.handle_exit = vt_op(handle_exit),
 	.skip_emulated_instruction = vmx_skip_emulated_instruction,
 	.update_emulated_instruction = vmx_update_emulated_instruction,
+	.unhandleable_emulation_required = vt_op(unhandleable_emulation_required),
 	.set_interrupt_shadow = vt_op(set_interrupt_shadow),
 	.get_interrupt_shadow = vt_op(get_interrupt_shadow),
 	.patch_hypercall = vt_op(patch_hypercall),

arch/x86/kvm/vmx/vmx.c

@@ -6034,7 +6034,7 @@ static int handle_nmi_window(struct kvm_vcpu *vcpu)
  * with unsrestricted guest mode disabled) and KVM can't faithfully emulate the
  * current vCPU state.
  */
-static bool vmx_unhandleable_emulation_required(struct kvm_vcpu *vcpu)
+bool vmx_unhandleable_emulation_required(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
@@ -6111,11 +6111,6 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
 
 int vmx_vcpu_pre_run(struct kvm_vcpu *vcpu)
 {
-	if (vmx_unhandleable_emulation_required(vcpu)) {
-		kvm_prepare_emulation_failure_exit(vcpu);
-		return 0;
-	}
-
 	return 1;
 }
 

arch/x86/kvm/vmx/x86_ops.h

@@ -31,6 +31,7 @@ int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath);
 void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu);
 int vmx_skip_emulated_instruction(struct kvm_vcpu *vcpu);
 void vmx_update_emulated_instruction(struct kvm_vcpu *vcpu);
+bool vmx_unhandleable_emulation_required(struct kvm_vcpu *vcpu);
 int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info);
 #ifdef CONFIG_KVM_SMM
 int vmx_smi_allowed(struct kvm_vcpu *vcpu, bool for_injection);

arch/x86/kvm/x86.c

@@ -8862,6 +8862,11 @@ static int kvm_x86_vcpu_pre_run(struct kvm_vcpu *vcpu)
 	    !kvm_apic_init_sipi_allowed(vcpu))
 		return -EINVAL;
 
+	if (kvm_x86_call(unhandleable_emulation_required)(vcpu)) {
+		kvm_prepare_emulation_failure_exit(vcpu);
+		return 0;
+	}
+
 	return kvm_x86_call(vcpu_pre_run)(vcpu);
 }