Re: x86: Spurious vectors not handled robustly

[Thomas Gleixner <tglx@linutronix.de>]

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.

Thanks,

	tglx