x86/kvm/vmx: Fix x86_64 CFI build

It was missed that idt_do_interrupt_irqoff() gets compiled on x84_64;
this is a problem for CFI builds because it includes an unadorned
indirect call. It is however completely dead code.

Rework things to not emit this function at all.

Fixes: 0701c9e ("x86/kvm/vmx: Move IRQ/NMI dispatch from KVM into x86 core")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Calvin Owens <calvin@wbinvd.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260526090631.GA4149641@noisy.programming.kicks-ass.net

Author: Peter Zijlstra

arch/x86/entry/common.c

@@ -55,7 +55,7 @@ noinstr void x86_entry_from_kvm(unsigned int event_type, unsigned int vector)
 	 * The FRED NMI context is significantly different and will not work
 	 * right (specifically FRED fixed the NMI recursion issue).
 	 */
-	idt_entry_from_kvm(vector);
+	idt_do_nmi_irqoff();
 }
 EXPORT_SYMBOL_FOR_KVM(x86_entry_from_kvm);
 #endif

arch/x86/entry/entry.S

@@ -109,11 +109,13 @@ EXPORT_SYMBOL(__ref_stack_chk_guard);
 	RET
 .endm
 
+#ifndef CONFIG_X86_64
 .pushsection .text, "ax"
 SYM_FUNC_START(idt_do_interrupt_irqoff)
 	IDT_DO_EVENT_IRQOFF CALL_NOSPEC _ASM_ARG1
 SYM_FUNC_END(idt_do_interrupt_irqoff)
 .popsection
+#endif
 
 .pushsection .noinstr.text, "ax"
 SYM_FUNC_START(idt_do_nmi_irqoff)

arch/x86/kernel/idt.c

@@ -268,18 +268,10 @@ void __init idt_setup_early_pf(void)
 }
 #endif
 
-#if IS_ENABLED(CONFIG_KVM_INTEL)
-noinstr void idt_entry_from_kvm(unsigned int vector)
+#if IS_ENABLED(CONFIG_KVM_INTEL) && !defined(CONFIG_X86_64)
+void idt_entry_from_kvm(unsigned int vector)
 {
-	if (vector == NMI_VECTOR)
-		return idt_do_nmi_irqoff();
-
-	/*
-	 * Only the NMI path requires noinstr.
-	 */
-	instrumentation_begin();
 	idt_do_interrupt_irqoff(gate_offset(idt_table + vector));
-	instrumentation_end();
 }
 #endif