Re: [PATCH] x86/idt: Simplify the idt_setup_apic_and_irq_gates()

[Dou Liyang]

Hi Thomas,

At 05/19/2018 08:32 PM, Thomas Gleixner wrote:
> On Thu, 26 Apr 2018, Dou Liyang wrote:
>
> > The vectors between FIRST_SYSTEM_VECTOR and NR_VECTORS are special IRQ
> > vectors used by the SMP architecture. But, if X86_LOCAL_APIC=n, it will
> > not be used, and the FIRST_SYSTEM_VECTOR is equal to NR_VECTORS.
>
> Correct, but that function has nothing to do with FIRST_SYSTEM_VECTOR.

Oops, sorry, when I reread, my changelog even made me misunderstand.
the patch hided it.

    void __init idt_setup_apic_and_irq_gates(void)
    ...
        for_each_clear_bit_from(i, system_vectors, FIRST_SYSTEM_VECTOR)
                                                   ^^^^^^^^^^^^^^^^^^^
    ...
        for_each_clear_bit_from(i, system_vectors, NR_VECTORS)

What I want to say is:

 In the APIC=n system, the FIRST_SYSTEM_VECTOR is equal to
 NR_VECTORS, So all entries has been set by

    for_each_clear_bit_from(i, system_vectors, FIRST_SYSTEM_VECTOR)
   Â...

 the following setup code for APIC=n is redundant. Just setup gates for
 APIC=y.

If it is OK, I will send v2.

Thanks,

	dou

> > diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
> > index 2c3a1b4294eb..8b4174890706 100644
> > --- a/arch/x86/kernel/idt.c
> > +++ b/arch/x86/kernel/idt.c
> > @@ -317,15 +317,16 @@ void __init idt_setup_apic_and_irq_gates(void)
> >   		set_intr_gate(i, entry);
> >   	}
> >   
> > -	for_each_clear_bit_from(i, system_vectors, NR_VECTORS) {
> > +	/*
> > +	 * If X86_LOCAL_APIC=n, the FIRST_SYSTEM_VECTOR is equal to NR_VECTORS
> > +	 * Just consider the X86_LOCAL_APIC=y case
> > +	 */
> >   #ifdef CONFIG_X86_LOCAL_APIC
> > +	for_each_clear_bit_from(i, system_vectors, NR_VECTORS) {
> >   		set_bit(i, system_vectors);
> >   		set_intr_gate(i, spurious_interrupt);
> > -#else
> > -		entry = irq_entries_start + 8 * (i - FIRST_EXTERNAL_VECTOR);
> > -		set_intr_gate(i, entry);
> > -#endif
> >   	}
> > +#endif
>
> That change breaks the LOCAL_APIC=n case in a subtle way. What the function
> does is to set _ALL_ entries starting from FIRST_EXTERNAL_VECTOR up to
> NR_VECTORS in the IDT to a known target, except those which are already
> occupied by a system vector.
>
> So for APIC=y this sets them to: spurious vector and for APIC=n it sets it
> to the corresppnding vector entry. You remove the latter...
>  > Thanks,
>
> 	tglx