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

[Florian Bezdeka <florian.bezdeka@siemens.com>]

On Tue, 2026-05-26 at 12:21 +0200, Peter Zijlstra wrote:
> 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.

Right. Thanks for the super fast response. Let me double check what is
going on here...