Re: x86: Spurious vectors not handled robustly

[Jan Kiszka <jan.kiszka@siemens.com>]

On 24.06.19 12:09, Thomas Gleixner wrote:
> Jan,
> 
> On Mon, 24 Jun 2019, Jan Kiszka wrote:
>> probably since "x86: Avoid building unused IRQ entry stubs" (2414e021ac8d),
>> the kernel can no longer tell spurious IRQs by the APIC apart from spuriously
>> triggered unused vectors.
> 
> Err. It does.
> 
>> We've managed to trigger such a cause with the Jailhouse hypervisor
>> (incorrectly injected MANAGED_IRQ_SHUTDOWN_VECTOR), and the result was
>> not only a misreport of the vector number (0xff instead of 0xef - took me
>> a while...), but also stalled interrupts of equal and lower priority
>> because a spurious interrupt is not (and must not be) acknowledged.
> 
> That does not make sense.
> 
> __visible void __irq_entry smp_spurious_interrupt(struct pt_regs *regs)
> {
>          u8 vector = ~regs->orig_ax;
>          u32 v;
> 
>          entering_irq();
>          trace_spurious_apic_entry(vector);
>          /*
>           * Check if this really is a spurious interrupt and ACK it
>           * if it is a vectored one.  Just in case...
>           */
>          v = apic_read(APIC_ISR + ((vector & ~0x1f) >> 1));
>          if (v & (1 << (vector & 0x1f)))
>                  ack_APIC_irq();
> 
> If it is a vectored one it _IS_ acked.
> 
>          inc_irq_stat(irq_spurious_count);
> 
>   	/* see sw-dev-man vol 3, chapter 7.4.13.5 */
>          pr_info("spurious APIC interrupt through vector %02x on CPU#%d, "
>                  "should never happen.\n", vector, smp_processor_id());
> 
> and the vector through which that comes is printed correctly, unless
> regs->orig_ax is hosed.

...which is exactly the case: Since that commit, all unused vectors share the 
same entry point, spurious_interrupt, see idt_setup_apic_and_irq_gates(). And 
that entry point sets orig_ax to ~0xff.

Jan

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