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

  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
>
>
>
>


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