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

> 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

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