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.. SPDX-License-Identifier: GPL-2.0
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===============
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Boot Interrupts
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===============
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:Author: - Sean V Kelley <sean.v.kelley@linux.intel.com>
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Overview
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========
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On PCI Express, interrupts are represented with either MSI or inbound
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interrupt messages (Assert_INTx/Deassert_INTx). The integrated IO-APIC in a
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given Core IO converts the legacy interrupt messages from PCI Express to
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MSI interrupts. If the IO-APIC is disabled (via the mask bits in the
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IO-APIC table entries), the messages are routed to the legacy PCH. This
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in-band interrupt mechanism was traditionally necessary for systems that
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did not support the IO-APIC and for boot. Intel in the past has used the
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term "boot interrupts" to describe this mechanism. Further, the PCI Express
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protocol describes this in-band legacy wire-interrupt INTx mechanism for
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I/O devices to signal PCI-style level interrupts. The subsequent paragraphs
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describe problems with the Core IO handling of INTx message routing to the
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PCH and mitigation within BIOS and the OS.
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Issue
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=====
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When in-band legacy INTx messages are forwarded to the PCH, they in turn
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trigger a new interrupt for which the OS likely lacks a handler. When an
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interrupt goes unhandled over time, they are tracked by the Linux kernel as
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Spurious Interrupts. The IRQ will be disabled by the Linux kernel after it
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reaches a specific count with the error "nobody cared". This disabled IRQ
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now prevents valid usage by an existing interrupt which may happen to share
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the IRQ line::
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irq 19: nobody cared (try booting with the "irqpoll" option)
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CPU: 0 PID: 2988 Comm: irq/34-nipalk Tainted: 4.14.87-rt49-02410-g4a640ec-dirty #1
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Hardware name: National Instruments NI PXIe-8880/NI PXIe-8880, BIOS 2.1.5f1 01/09/2020
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Call Trace:
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<IRQ>
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? dump_stack+0x46/0x5e
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? __report_bad_irq+0x2e/0xb0
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? note_interrupt+0x242/0x290
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? nNIKAL100_memoryRead16+0x8/0x10 [nikal]
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? handle_irq_event_percpu+0x55/0x70
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? handle_irq_event+0x4f/0x80
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? handle_fasteoi_irq+0x81/0x180
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? handle_irq+0x1c/0x30
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? do_IRQ+0x41/0xd0
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? common_interrupt+0x84/0x84
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</IRQ>
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handlers:
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irq_default_primary_handler threaded usb_hcd_irq
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Disabling IRQ #19
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Conditions
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==========
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The use of threaded interrupts is the most likely condition to trigger
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this problem today. Threaded interrupts may not be reenabled after the IRQ
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handler wakes. These "one shot" conditions mean that the threaded interrupt
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needs to keep the interrupt line masked until the threaded handler has run.
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Especially when dealing with high data rate interrupts, the thread needs to
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run to completion; otherwise some handlers will end up in stack overflows
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since the interrupt of the issuing device is still active.
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Affected Chipsets
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=================
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The legacy interrupt forwarding mechanism exists today in a number of
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devices including but not limited to chipsets from AMD/ATI, Broadcom, and
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Intel. Changes made through the mitigations below have been applied to
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drivers/pci/quirks.c
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Starting with ICX there are no longer any IO-APICs in the Core IO's
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devices. IO-APIC is only in the PCH. Devices connected to the Core IO's
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PCIe Root Ports will use native MSI/MSI-X mechanisms.
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Mitigations
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===========
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The mitigations take the form of PCI quirks. The preference has been to
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first identify and make use of a means to disable the routing to the PCH.
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In such a case a quirk to disable boot interrupt generation can be
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added. [1]_
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Intel® 6300ESB I/O Controller Hub
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Alternate Base Address Register:
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BIE: Boot Interrupt Enable
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== ===========================
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0 Boot interrupt is enabled.
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1 Boot interrupt is disabled.
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== ===========================
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Intel® Sandy Bridge through Sky Lake based Xeon servers:
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Coherent Interface Protocol Interrupt Control
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dis_intx_route2pch/dis_intx_route2ich/dis_intx_route2dmi2:
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When this bit is set. Local INTx messages received from the
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Intel® Quick Data DMA/PCI Express ports are not routed to legacy
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PCH - they are either converted into MSI via the integrated IO-APIC
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(if the IO-APIC mask bit is clear in the appropriate entries)
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or cause no further action (when mask bit is set)
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In the absence of a way to directly disable the routing, another approach
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has been to make use of PCI Interrupt pin to INTx routing tables for
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purposes of redirecting the interrupt handler to the rerouted interrupt
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line by default. Therefore, on chipsets where this INTx routing cannot be
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disabled, the Linux kernel will reroute the valid interrupt to its legacy
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interrupt. This redirection of the handler will prevent the occurrence of
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the spurious interrupt detection which would ordinarily disable the IRQ
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line due to excessive unhandled counts. [2]_
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The config option X86_REROUTE_FOR_BROKEN_BOOT_IRQS exists to enable (or
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disable) the redirection of the interrupt handler to the PCH interrupt
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line. The option can be overridden by either pci=ioapicreroute or
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pci=noioapicreroute. [3]_
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More Documentation
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==================
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There is an overview of the legacy interrupt handling in several datasheets
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(6300ESB and 6700PXH below). While largely the same, it provides insight
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into the evolution of its handling with chipsets.
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Example of disabling of the boot interrupt
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------------------------------------------
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- Intel® 6300ESB I/O Controller Hub (Document # 300641-004US)
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5.7.3 Boot Interrupt
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https://www.intel.com/content/dam/doc/datasheet/6300esb-io-controller-hub-datasheet.pdf
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- Intel® Xeon® Processor E5-1600/2400/2600/4600 v3 Product Families
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Datasheet - Volume 2: Registers (Document # 330784-003)
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6.6.41 cipintrc Coherent Interface Protocol Interrupt Control
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https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/xeon-e5-v3-datasheet-vol-2.pdf
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Example of handler rerouting
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----------------------------
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- Intel® 6700PXH 64-bit PCI Hub (Document # 302628)
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2.15.2 PCI Express Legacy INTx Support and Boot Interrupt
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https://www.intel.com/content/dam/doc/datasheet/6700pxh-64-bit-pci-hub-datasheet.pdf
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If you have any legacy PCI interrupt questions that aren't answered, email me.
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Cheers,
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Sean V Kelley
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sean.v.kelley@linux.intel.com
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.. [1] https://lore.kernel.org/r/12131949181903-git-send-email-sassmann@suse.de/
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.. [2] https://lore.kernel.org/r/12131949182094-git-send-email-sassmann@suse.de/
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.. [3] https://lore.kernel.org/r/487C8EA7.6020205@suse.de/
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