Documentation/PCI/boot-interrupts.rst - linux - Git at Google

 .. 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 reenabled 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/
	.. 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 reenabled 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/