KVM: nVMX: Allow the caller to provide instruction length on nested VM-Exit

Rework the nested VM-Exit helper to take the instruction length as a
parameter, and convert nested_vmx_vmexit() into a "default" wrapper that
grabs the length from vmcs02 as appropriate.  This will allow KVM to set
the correct instruction length when synthesizing a nested VM-Exit when
emulating an instruction that L1 wants to intercept.

No functional change intended, as the path to prepare_vmcs12()'s reading
of vmcs02.VM_EXIT_INSTRUCTION_LEN is gated on the same set of conditions
as the VMREAD in the new nested_vmx_vmexit().

Link: https://lore.kernel.org/r/20250201015518.689704-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/nested.c

@@ -4618,7 +4618,7 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
  */
 static void prepare_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 			   u32 vm_exit_reason, u32 exit_intr_info,
-			   unsigned long exit_qualification)
+			   unsigned long exit_qualification, u32 exit_insn_len)
 {
 	/* update exit information fields: */
 	vmcs12->vm_exit_reason = vm_exit_reason;
@@ -4646,7 +4646,7 @@ static void prepare_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 					  vm_exit_reason, exit_intr_info);
 
 		vmcs12->vm_exit_intr_info = exit_intr_info;
-		vmcs12->vm_exit_instruction_len = vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
+		vmcs12->vm_exit_instruction_len = exit_insn_len;
 		vmcs12->vmx_instruction_info = vmcs_read32(VMX_INSTRUCTION_INFO);
 
 		/*
@@ -4930,8 +4930,9 @@ static void nested_vmx_restore_host_state(struct kvm_vcpu *vcpu)
  * and modify vmcs12 to make it see what it would expect to see there if
  * L2 was its real guest. Must only be called when in L2 (is_guest_mode())
  */
-void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
-		       u32 exit_intr_info, unsigned long exit_qualification)
+void __nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
+			 u32 exit_intr_info, unsigned long exit_qualification,
+			 u32 exit_insn_len)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
@@ -4981,7 +4982,8 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 
 		if (vm_exit_reason != -1)
 			prepare_vmcs12(vcpu, vmcs12, vm_exit_reason,
-				       exit_intr_info, exit_qualification);
+				       exit_intr_info, exit_qualification,
+				       exit_insn_len);
 
 		/*
 		 * Must happen outside of sync_vmcs02_to_vmcs12() as it will

arch/x86/kvm/vmx/nested.h

@@ -26,8 +26,26 @@ void nested_vmx_free_vcpu(struct kvm_vcpu *vcpu);
 enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 						     bool from_vmentry);
 bool nested_vmx_reflect_vmexit(struct kvm_vcpu *vcpu);
-void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
-		       u32 exit_intr_info, unsigned long exit_qualification);
+void __nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
+			 u32 exit_intr_info, unsigned long exit_qualification,
+			 u32 exit_insn_len);
+
+static inline void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
+				     u32 exit_intr_info,
+				     unsigned long exit_qualification)
+{
+	u32 exit_insn_len;
+
+	if (to_vmx(vcpu)->fail || vm_exit_reason == -1 ||
+	    (vm_exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
+		exit_insn_len = 0;
+	else
+		exit_insn_len = vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
+
+	__nested_vmx_vmexit(vcpu, vm_exit_reason, exit_intr_info,
+			    exit_qualification, exit_insn_len);
+}
+
 void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu);
 int vmx_set_vmx_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data);
 int vmx_get_vmx_msr(struct nested_vmx_msrs *msrs, u32 msr_index, u64 *pdata);