Re: [PATCH v2 1/2] x86/kvm/vmx: Move IRQ/NMI dispatch from KVM into x86 core

[Yan Zhao <yan.y.zhao@intel.com>]

On Fri, May 01, 2026 at 10:37:17PM +0200, Peter Zijlstra wrote:
> --- /dev/null
> +++ b/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);
FYI:
I can met below error with "make ARCH=i386 allnoconfig" or
"make ARCH=x86_64 allnoconfig".

ld: vmlinux.o: in function `idt_entry_from_kvm':
(.noinstr.text+0x1618): undefined reference to `idt_do_interrupt_irqoff'
ld: (.noinstr.text+0x161f): undefined reference to `idt_do_nmi_irqoff'
make[2]: *** [scripts/Makefile.vmlinux:72: vmlinux.unstripped] Error 1
make[1]: *** [/home/yan/kernel/tdx-upstream/tdx-kvm-upstream/Makefile:1335: vmlinux] Error 2
make: *** [Makefile:248: __sub-make] Error 2

So, I added below fix:

diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index 0e8fb61f63ff..7bcf1decc034 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -268,6 +268,7 @@ 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)
@@ -280,6 +281,7 @@ noinstr void idt_entry_from_kvm(unsigned int vector)
        idt_do_interrupt_irqoff(gate_offset(idt_table + vector));
        instrumentation_end();
 }
+#endif

 static void __init idt_map_in_cea(void)
 {


> +#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 (speficially FRED fixed the NMI recursion issue).
> +	 */
> +	idt_entry_from_kvm(vector);
> +}
> +EXPORT_SYMBOL_FOR_KVM(x86_entry_from_kvm);
> +#endif