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Date:   Mon, 24 Jun 2019 12:09:28 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jan Kiszka <jan.kiszka@siemens.com>
cc:     Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: x86: Spurious vectors not handled robustly
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References: <e525108f-3749-4e1d-1ac2-0d0a2655f15f@siemens.com>
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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