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SubjectRe: x86: Spurious vectors not handled robustly
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

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