Re: [PATCH v3 1/2] x86/kvm/vmx: Move IRQ/NMI dispatch from KVM into x86 core

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 26, 2026 at 10:01:41AM +0000, Bezdeka, Florian wrote:

> > +#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);
> 
> Seems this landed in 7.1-rc5.
> 
> I'm seeing a build failure here:
> 
> arch/x86/entry/common.c: In function ‘x86_entry_from_kvm’:
> arch/x86/entry/common.c:27:17: error: implicit declaration of function ‘fred_entry_from_kvm’; did you mean ‘idt_entry_from_kvm’? [-Wimplicit-function-declaration]
>    27 |                 fred_entry_from_kvm(event_type, vector);
>       |                 ^~~~~~~~~~~~~~~~~~~
>       |                 idt_entry_from_kvm
> arch/x86/entry/common.c:50:24: error: ‘return’ with a value, in function returning void [-Wreturn-mismatch]
>    50 |                 return fred_entry_from_kvm(event_type, vector);
>       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/entry/common.c:18:14: note: declared here
>    18 | noinstr void x86_entry_from_kvm(unsigned int event_type, unsigned int 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 (speficially FRED fixed the NMI recursion issue).
> > +	 */
> > +	idt_entry_from_kvm(vector);
> > +}
> > +EXPORT_SYMBOL_FOR_KVM(x86_entry_from_kvm);
> > +#endif
> > 
> 
> [snip]
> 
> > --- a/arch/x86/include/asm/entry-common.h
> > +++ b/arch/x86/include/asm/entry-common.h
> > @@ -97,4 +97,6 @@ static __always_inline void arch_exit_to
> >  }
> >  #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
> > --- a/arch/x86/include/asm/fred.h
> > +++ b/arch/x86/include/asm/fred.h
> > @@ -110,7 +110,6 @@ static __always_inline unsigned long fre
> >  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) { }
> 
> That seems still necessary for the !CONFIG_X86_FRED case.

The thing is, KVM_INTEL should force X86_FRED, I'm not sure how you can
have both KVM_INTEL and !X86_FRED, that should be an invalid config.