.. SPDX-License-Identifier: GPL-2.0

===============
Boot Interrupts
===============

:Author: - Sean V Kelley <sean.v.kelley@linux.intel.com>

Overview
========

On PCI Express, interrupts are represented with either MSI or inbound
interrupt messages (Assert_INTx/Deassert_INTx). The integrated IO-APIC in a
given Core IO converts the legacy interrupt messages from PCI Express to
MSI interrupts.  If the IO-APIC is disabled (via the mask bits in the
IO-APIC table entries), the messages are routed to the legacy PCH. This
in-band interrupt mechanism was traditionally necessary for systems that
did not support the IO-APIC and for boot. Intel in the past has used the
term "boot interrupts" to describe this mechanism. Further, the PCI Express
protocol describes this in-band legacy wire-interrupt INTx mechanism for
I/O devices to signal PCI-style level interrupts. The subsequent paragraphs
describe problems with the Core IO handling of INTx message routing to the
PCH and mitigation within BIOS and the OS.


Issue
=====

When in-band legacy INTx messages are forwarded to the PCH, they in turn
trigger a new interrupt for which the OS likely lacks a handler. When an
interrupt goes unhandled over time, they are tracked by the Linux kernel as
Spurious Interrupts. The IRQ will be disabled by the Linux kernel after it
reaches a specific count with the error "nobody cared". This disabled IRQ
now prevents valid usage by an existing interrupt which may happen to share
the IRQ line::

  irq 19: nobody cared (try booting with the "irqpoll" option)
  CPU: 0 PID: 2988 Comm: irq/34-nipalk Tainted: 4.14.87-rt49-02410-g4a640ec-dirty #1
  Hardware name: National Instruments NI PXIe-8880/NI PXIe-8880, BIOS 2.1.5f1 01/09/2020
  Call Trace:

  <IRQ>
   ? dump_stack+0x46/0x5e
   ? __report_bad_irq+0x2e/0xb0
   ? note_interrupt+0x242/0x290
   ? nNIKAL100_memoryRead16+0x8/0x10 [nikal]
   ? handle_irq_event_percpu+0x55/0x70
   ? handle_irq_event+0x4f/0x80
   ? handle_fasteoi_irq+0x81/0x180
   ? handle_irq+0x1c/0x30
   ? do_IRQ+0x41/0xd0
   ? common_interrupt+0x84/0x84
  </IRQ>

  handlers:
  irq_default_primary_handler threaded usb_hcd_irq
  Disabling IRQ #19


Conditions
==========

The use of threaded interrupts is the most likely condition to trigger
this problem today. Threaded interrupts may not be re-enabled after the IRQ
handler wakes. These "one shot" conditions mean that the threaded interrupt
needs to keep the interrupt line masked until the threaded handler has run.
Especially when dealing with high data rate interrupts, the thread needs to
run to completion; otherwise some handlers will end up in stack overflows
since the interrupt of the issuing device is still active.

Affected Chipsets
=================

The legacy interrupt forwarding mechanism exists today in a number of
devices including but not limited to chipsets from AMD/ATI, Broadcom, and
Intel. Changes made through the mitigations below have been applied to
drivers/pci/quirks.c

Starting with ICX there are no longer any IO-APICs in the Core IO's
devices.  IO-APIC is only in the PCH.  Devices connected to the Core IO's
PCIe Root Ports will use native MSI/MSI-X mechanisms.

Mitigations
===========

The mitigations take the form of PCI quirks. The preference has been to
first identify and make use of a means to disable the routing to the PCH.
In such a case a quirk to disable boot interrupt generation can be
added. [1]_

Intel® 6300ESB I/O Controller Hub
  Alternate Base Address Register:
   BIE: Boot Interrupt Enable

	  ==  ===========================
	  0   Boot interrupt is enabled.
	  1   Boot interrupt is disabled.
	  ==  ===========================

Intel® Sandy Bridge through Sky Lake based Xeon servers:
  Coherent Interface Protocol Interrupt Control
   dis_intx_route2pch/dis_intx_route2ich/dis_intx_route2dmi2:
	  When this bit is set. Local INTx messages received from the
	  Intel® Quick Data DMA/PCI Express ports are not routed to legacy
	  PCH - they are either converted into MSI via the integrated IO-APIC
	  (if the IO-APIC mask bit is clear in the appropriate entries)
	  or cause no further action (when mask bit is set)

In the absence of a way to directly disable the routing, another approach
has been to make use of PCI Interrupt pin to INTx routing tables for
purposes of redirecting the interrupt handler to the rerouted interrupt
line by default.  Therefore, on chipsets where this INTx routing cannot be
disabled, the Linux kernel will reroute the valid interrupt to its legacy
interrupt. This redirection of the handler will prevent the occurrence of
the spurious interrupt detection which would ordinarily disable the IRQ
line due to excessive unhandled counts. [2]_

The config option X86_REROUTE_FOR_BROKEN_BOOT_IRQS exists to enable (or
disable) the redirection of the interrupt handler to the PCH interrupt
line. The option can be overridden by either pci=ioapicreroute or
pci=noioapicreroute. [3]_


More Documentation
==================

There is an overview of the legacy interrupt handling in several datasheets
(6300ESB and 6700PXH below). While largely the same, it provides insight
into the evolution of its handling with chipsets.

Example of disabling of the boot interrupt
------------------------------------------

      - Intel® 6300ESB I/O Controller Hub (Document # 300641-004US)
	5.7.3 Boot Interrupt
	https://www.intel.com/content/dam/doc/datasheet/6300esb-io-controller-hub-datasheet.pdf

      - Intel® Xeon® Processor E5-1600/2400/2600/4600 v3 Product Families
	Datasheet - Volume 2: Registers (Document # 330784-003)
	6.6.41 cipintrc Coherent Interface Protocol Interrupt Control
	https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/xeon-e5-v3-datasheet-vol-2.pdf

Example of handler rerouting
----------------------------

      - Intel® 6700PXH 64-bit PCI Hub (Document # 302628)
	2.15.2 PCI Express Legacy INTx Support and Boot Interrupt
	https://www.intel.com/content/dam/doc/datasheet/6700pxh-64-bit-pci-hub-datasheet.pdf


If you have any legacy PCI interrupt questions that aren't answered, email me.

Cheers,
    Sean V Kelley
    sean.v.kelley@linux.intel.com

.. [1] https://lore.kernel.org/r/12131949181903-git-send-email-sassmann@suse.de/
.. [2] https://lore.kernel.org/r/12131949182094-git-send-email-sassmann@suse.de/
.. [3] https://lore.kernel.org/r/487C8EA7.6020205@suse.de/