x86/kvm/vmx: Move IRQ/NMI dispatch from KVM into x86 core

Move the VMX interrupt dispatch magic into the x86 core code. This
isolates KVM from the FRED/IDT decisions and reduces the amount of
EXPORT_SYMBOL_FOR_KVM().

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: "Verma, Vishal L" <vishal.l.verma@intel.com>
Tested-by: Zhao Liu <zhao1.liu@intel.com>
Tested-by: Zhao Liu <zhao1.liu@intel.com>
Tested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linxu.intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20260508091829.GO3126523@noisy.programming.kicks-ass.net

Author: Peter Zijlstra

arch/x86/entry/Makefile

@@ -13,7 +13,7 @@ CFLAGS_REMOVE_syscall_64.o	= $(CC_FLAGS_FTRACE)
 CFLAGS_syscall_32.o		+= -fno-stack-protector
 CFLAGS_syscall_64.o		+= -fno-stack-protector
 
-obj-y				:= entry.o entry_$(BITS).o syscall_$(BITS).o
+obj-y				:= entry.o entry_$(BITS).o syscall_$(BITS).o common.o
 
 obj-y				+= vdso/
 obj-y				+= vsyscall/

arch/x86/entry/common.c

@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/entry-common.h>
+#include <linux/kvm_types.h>
+#include <asm/fred.h>
+#include <asm/desc.h>
+
+#if IS_ENABLED(CONFIG_KVM_INTEL)
+/*
+ * On VMX, NMIs and IRQs (as configured by KVM) are acknowledged by hardware as
+ * part of the VM-Exit, i.e. the event itself is consumed as part the VM-Exit.
+ * x86_entry_from_kvm() is invoked by KVM to effectively forward NMIs and IRQs
+ * to the kernel for servicing.  On SVM, a.k.a. AMD, the NMI/IRQ VM-Exit is
+ * purely a signal that an NMI/IRQ is pending, i.e. the event that triggered
+ * the VM-Exit is held pending until it's unblocked in the host.
+ */
+noinstr void x86_entry_from_kvm(unsigned int event_type, unsigned int vector)
+{
+	if (event_type == EVENT_TYPE_EXTINT) {
+#ifdef CONFIG_X86_64
+		/*
+		 * Use FRED dispatch, even when running IDT. The dispatch
+		 * tables are kept in sync between FRED and IDT, and the FRED
+		 * dispatch works well with CFI.
+		 */
+		fred_entry_from_kvm(event_type, vector);
+#else
+		idt_entry_from_kvm(vector);
+#endif
+		return;
+	}
+
+	WARN_ON_ONCE(event_type != EVENT_TYPE_NMI);
+
+#ifdef CONFIG_X86_64
+	if (cpu_feature_enabled(X86_FEATURE_FRED))
+		return fred_entry_from_kvm(event_type, vector);
+#endif
+
+	/*
+	 * Notably, we must use IDT dispatch for NMI when running in IDT mode.
+	 * The FRED NMI context is significantly different and will not work
+	 * right (specifically FRED fixed the NMI recursion issue).
+	 */
+	idt_entry_from_kvm(vector);
+}
+EXPORT_SYMBOL_FOR_KVM(x86_entry_from_kvm);
+#endif

arch/x86/entry/entry.S

@@ -75,3 +75,49 @@ THUNK warn_thunk_thunk, __warn_thunk
 #if defined(CONFIG_STACKPROTECTOR) && defined(CONFIG_SMP)
 EXPORT_SYMBOL(__ref_stack_chk_guard);
 #endif
+
+#if IS_ENABLED(CONFIG_KVM_INTEL)
+.macro IDT_DO_EVENT_IRQOFF call_insn call_target
+	/*
+	 * Unconditionally create a stack frame, getting the correct RSP on the
+	 * stack (for x86-64) would take two instructions anyways, and RBP can
+	 * be used to restore RSP to make objtool happy (see below).
+	 */
+	push %_ASM_BP
+	mov %_ASM_SP, %_ASM_BP
+
+#ifdef CONFIG_X86_64
+	/*
+	 * Align RSP to a 16-byte boundary (to emulate CPU behavior) before
+	 * creating the synthetic interrupt stack frame for the IRQ/NMI.
+	 */
+	and  $-16, %rsp
+	push $__KERNEL_DS
+	push %rbp
+#endif
+	pushf
+	push $__KERNEL_CS
+	\call_insn \call_target
+
+	/*
+	 * "Restore" RSP from RBP, even though IRET has already unwound RSP to
+	 * the correct value.  objtool doesn't know the callee will IRET and,
+	 * without the explicit restore, thinks the stack is getting walloped.
+	 * Using an unwind hint is problematic due to x86-64's dynamic alignment.
+	 */
+	leave
+	RET
+.endm
+
+.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
+
+.pushsection .noinstr.text, "ax"
+SYM_FUNC_START(idt_do_nmi_irqoff)
+	IDT_DO_EVENT_IRQOFF call asm_exc_nmi_kvm_vmx
+SYM_FUNC_END(idt_do_nmi_irqoff)
+.popsection
+#endif

arch/x86/entry/entry_64_fred.S

@@ -147,5 +147,4 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
 	RET
 
 SYM_FUNC_END(asm_fred_entry_from_kvm)
-EXPORT_SYMBOL_FOR_KVM(asm_fred_entry_from_kvm);
 #endif

arch/x86/include/asm/desc.h

@@ -438,6 +438,10 @@ extern void idt_setup_traps(void);
 extern void idt_setup_apic_and_irq_gates(void);
 extern bool idt_is_f00f_address(unsigned long address);
 
+extern void idt_do_interrupt_irqoff(unsigned long address);
+extern void idt_do_nmi_irqoff(void);
+extern void idt_entry_from_kvm(unsigned int vector);
+
 #ifdef CONFIG_X86_64
 extern void idt_setup_early_pf(void);
 #else

arch/x86/include/asm/desc_defs.h

@@ -145,7 +145,7 @@ struct gate_struct {
 typedef struct gate_struct gate_desc;
 
 #ifndef _SETUP
-static inline unsigned long gate_offset(const gate_desc *g)
+static __always_inline unsigned long gate_offset(const gate_desc *g)
 {
 #ifdef CONFIG_X86_64
 	return g->offset_low | ((unsigned long)g->offset_middle << 16) |

arch/x86/include/asm/entry-common.h

@@ -97,4 +97,6 @@ static __always_inline void arch_exit_to_user_mode(void)
 }
 #define arch_exit_to_user_mode arch_exit_to_user_mode
 
+extern void x86_entry_from_kvm(unsigned int entry_type, unsigned int vector);
+
 #endif

arch/x86/include/asm/fred.h

@@ -110,7 +110,6 @@ static __always_inline unsigned long fred_event_data(struct pt_regs *regs) { ret
 static inline void cpu_init_fred_exceptions(void) { }
 static inline void cpu_init_fred_rsps(void) { }
 static inline void fred_complete_exception_setup(void) { }
-static inline void fred_entry_from_kvm(unsigned int type, unsigned int vector) { }
 static inline void fred_sync_rsp0(unsigned long rsp0) { }
 static inline void fred_update_rsp0(void) { }
 #endif /* CONFIG_X86_FRED */

arch/x86/kernel/idt.c

@@ -268,6 +268,21 @@ void __init idt_setup_early_pf(void)
 }
 #endif
 
+#if IS_ENABLED(CONFIG_KVM_INTEL)
+noinstr 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
+
 static void __init idt_map_in_cea(void)
 {
 	/*

arch/x86/kernel/nmi.c

@@ -614,7 +614,6 @@ DEFINE_IDTENTRY_RAW(exc_nmi_kvm_vmx)
 {
 	exc_nmi(regs);
 }
-EXPORT_SYMBOL_FOR_KVM(asm_exc_nmi_kvm_vmx);
 #endif
 
 #ifdef CONFIG_NMI_CHECK_CPU