Re: [PATCH] arch/x86/entry_fred: don't set up KVM IRQs if KVM is disabled

[Xin Li]

On 2/15/2024 10:31 PM, Paolo Bonzini wrote:
> On 2/16/24 03:10, Xin Li wrote:
> > On 2/15/2024 11:55 AM, Sean Christopherson wrote:
> > > +Paolo and Stephen
> > >
> > > FYI, there's a build failure in -next due to a collision between 
> > > kvm/next and
> > > tip/x86/fred.  The above makes everything happy.
> > >
> > > On Thu, Feb 15, 2024, Max Kellermann wrote:
> > > > When KVM is disabled, the POSTED_INTR_* macros do not exist, and the
> > > > build fails.
> > > >
> > > > Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
> > > > Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
> > > > ---
> > > >   arch/x86/entry/entry_fred.c | 2 ++
> > > >   1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
> > > > index ac120cbdaaf2..660b7f7f9a79 100644
> > > > --- a/arch/x86/entry/entry_fred.c
> > > > +++ b/arch/x86/entry/entry_fred.c
> > > > @@ -114,9 +114,11 @@ static idtentry_t 
> > > > sysvec_table[NR_SYSTEM_VECTORS] __ro_after_init = {
> > > >       SYSVEC(IRQ_WORK_VECTOR,            irq_work),
> > > > +#if IS_ENABLED(CONFIG_KVM)
> > > >       SYSVEC(POSTED_INTR_VECTOR,        kvm_posted_intr_ipi),
> > > >       SYSVEC(POSTED_INTR_WAKEUP_VECTOR,    kvm_posted_intr_wakeup_ipi),
> > > >       SYSVEC(POSTED_INTR_NESTED_VECTOR,    kvm_posted_intr_nested_ipi),
> > > > +#endif
> > > >   };
> > > >   static bool fred_setup_done __initdata;
> > > > --
> > > > 2.39.2
> >
> > We want to minimize #ifdeffery (which is why we didn't add any to
> > sysvec_table[]), would it be better to simply remove "#if 
> > IS_ENABLED(CONFIG_KVM)" around the the POSTED_INTR_* macros from the
> > Linux-next tree?
> >
> > BTW, kvm_posted_intr_*() are defined to NULL if !IS_ENABLED(CONFIG_KVM).
>
> It is intentional that KVM-related things are compiled out completely
> if !IS_ENABLED(CONFIG_KVM), 

In arch/x86/include/asm/irq_vectors.h, most vector definitions are not
under any #ifdeffery, e.g., THERMAL_APIC_VECTOR not under
CONFIG_X86_THERMAL_VECTOR and IRQ_WORK_VECTOR not under CONFIG_IRQ_WORK.

We'd better make all of them consistent, and the question is that should
we add #ifdefs or not.

> because then it's also not necessary to have
>
> # define fred_sysvec_kvm_posted_intr_ipi                NULL
> # define fred_sysvec_kvm_posted_intr_wakeup_ipi         NULL
> # define fred_sysvec_kvm_posted_intr_nested_ipi         NULL
>
> in arch/x86/include/asm/idtentry.h. The full conflict resultion is
>
> diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
> index ac120cbdaaf2..660b7f7f9a79 100644
> --- a/arch/x86/entry/entry_fred.c
> +++ b/arch/x86/entry/entry_fred.c
> @@ -114,9 +114,11 @@ static idtentry_t sysvec_table[NR_SYSTEM_VECTORS] 
> __ro_after_init = {
>
>       SYSVEC(IRQ_WORK_VECTOR,            irq_work),
>
> +#if IS_ENABLED(CONFIG_KVM)
>       SYSVEC(POSTED_INTR_VECTOR,        kvm_posted_intr_ipi),
>       SYSVEC(POSTED_INTR_WAKEUP_VECTOR,    kvm_posted_intr_wakeup_ipi),
>       SYSVEC(POSTED_INTR_NESTED_VECTOR,    kvm_posted_intr_nested_ipi),
> +#endif
>   };
>
>   static bool fred_setup_done __initdata;
> diff --git a/arch/x86/include/asm/idtentry.h 
> b/arch/x86/include/asm/idtentry.h
> index 749c7411d2f1..758f6a2838a8 100644
> --- a/arch/x86/include/asm/idtentry.h
> +++ b/arch/x86/include/asm/idtentry.h
> @@ -745,10 +745,6 @@ DECLARE_IDTENTRY_SYSVEC(IRQ_WORK_VECTOR,        
> sysvec_irq_work);
>   DECLARE_IDTENTRY_SYSVEC(POSTED_INTR_VECTOR,        
> sysvec_kvm_posted_intr_ipi);
>   DECLARE_IDTENTRY_SYSVEC(POSTED_INTR_WAKEUP_VECTOR,    
> sysvec_kvm_posted_intr_wakeup_ipi);
>   DECLARE_IDTENTRY_SYSVEC(POSTED_INTR_NESTED_VECTOR,    
> sysvec_kvm_posted_intr_nested_ipi);
> -#else
> -# define fred_sysvec_kvm_posted_intr_ipi        NULL
> -# define fred_sysvec_kvm_posted_intr_wakeup_ipi        NULL
> -# define fred_sysvec_kvm_posted_intr_nested_ipi        NULL
>   #endif
>
>   #if IS_ENABLED(CONFIG_HYPERV)
>
> and it seems to be a net improvement to me.  The #ifs match in
> the .h and .c files, and there are no unnecessary initializers
> in the sysvec_table.

I somehow get an impression that the x86 maintainers don't like #ifs in
the .c files, but I could be just wrong.

Thanks!
    Xin