1.. SPDX-License-Identifier: GPL-2.0 2 3=============== 4Boot Interrupts 5=============== 6 7:Author: - Sean V Kelley <sean.v.kelley@linux.intel.com> 8 9Overview 10======== 11 12On PCI Express, interrupts are represented with either MSI or inbound 13interrupt messages (Assert_INTx/Deassert_INTx). The integrated IO-APIC in a 14given Core IO converts the legacy interrupt messages from PCI Express to 15MSI interrupts. If the IO-APIC is disabled (via the mask bits in the 16IO-APIC table entries), the messages are routed to the legacy PCH. This 17in-band interrupt mechanism was traditionally necessary for systems that 18did not support the IO-APIC and for boot. Intel in the past has used the 19term "boot interrupts" to describe this mechanism. Further, the PCI Express 20protocol describes this in-band legacy wire-interrupt INTx mechanism for 21I/O devices to signal PCI-style level interrupts. The subsequent paragraphs 22describe problems with the Core IO handling of INTx message routing to the 23PCH and mitigation within BIOS and the OS. 24 25 26Issue 27===== 28 29When in-band legacy INTx messages are forwarded to the PCH, they in turn 30trigger a new interrupt for which the OS likely lacks a handler. When an 31interrupt goes unhandled over time, they are tracked by the Linux kernel as 32Spurious Interrupts. The IRQ will be disabled by the Linux kernel after it 33reaches a specific count with the error "nobody cared". This disabled IRQ 34now prevents valid usage by an existing interrupt which may happen to share 35the IRQ line:: 36 37 irq 19: nobody cared (try booting with the "irqpoll" option) 38 CPU: 0 PID: 2988 Comm: irq/34-nipalk Tainted: 4.14.87-rt49-02410-g4a640ec-dirty #1 39 Hardware name: National Instruments NI PXIe-8880/NI PXIe-8880, BIOS 2.1.5f1 01/09/2020 40 Call Trace: 41 42 <IRQ> 43 ? dump_stack+0x46/0x5e 44 ? __report_bad_irq+0x2e/0xb0 45 ? note_interrupt+0x242/0x290 46 ? nNIKAL100_memoryRead16+0x8/0x10 [nikal] 47 ? handle_irq_event_percpu+0x55/0x70 48 ? handle_irq_event+0x4f/0x80 49 ? handle_fasteoi_irq+0x81/0x180 50 ? handle_irq+0x1c/0x30 51 ? do_IRQ+0x41/0xd0 52 ? common_interrupt+0x84/0x84 53 </IRQ> 54 55 handlers: 56 irq_default_primary_handler threaded usb_hcd_irq 57 Disabling IRQ #19 58 59 60Conditions 61========== 62 63The use of threaded interrupts is the most likely condition to trigger 64this problem today. Threaded interrupts may not be re-enabled after the IRQ 65handler wakes. These "one shot" conditions mean that the threaded interrupt 66needs to keep the interrupt line masked until the threaded handler has run. 67Especially when dealing with high data rate interrupts, the thread needs to 68run to completion; otherwise some handlers will end up in stack overflows 69since the interrupt of the issuing device is still active. 70 71Affected Chipsets 72================= 73 74The legacy interrupt forwarding mechanism exists today in a number of 75devices including but not limited to chipsets from AMD/ATI, Broadcom, and 76Intel. Changes made through the mitigations below have been applied to 77drivers/pci/quirks.c 78 79Starting with ICX there are no longer any IO-APICs in the Core IO's 80devices. IO-APIC is only in the PCH. Devices connected to the Core IO's 81PCIe Root Ports will use native MSI/MSI-X mechanisms. 82 83Mitigations 84=========== 85 86The mitigations take the form of PCI quirks. The preference has been to 87first identify and make use of a means to disable the routing to the PCH. 88In such a case a quirk to disable boot interrupt generation can be 89added. [1]_ 90 91Intel® 6300ESB I/O Controller Hub 92 Alternate Base Address Register: 93 BIE: Boot Interrupt Enable 94 95 == =========================== 96 0 Boot interrupt is enabled. 97 1 Boot interrupt is disabled. 98 == =========================== 99 100Intel® Sandy Bridge through Sky Lake based Xeon servers: 101 Coherent Interface Protocol Interrupt Control 102 dis_intx_route2pch/dis_intx_route2ich/dis_intx_route2dmi2: 103 When this bit is set. Local INTx messages received from the 104 Intel® Quick Data DMA/PCI Express ports are not routed to legacy 105 PCH - they are either converted into MSI via the integrated IO-APIC 106 (if the IO-APIC mask bit is clear in the appropriate entries) 107 or cause no further action (when mask bit is set) 108 109In the absence of a way to directly disable the routing, another approach 110has been to make use of PCI Interrupt pin to INTx routing tables for 111purposes of redirecting the interrupt handler to the rerouted interrupt 112line by default. Therefore, on chipsets where this INTx routing cannot be 113disabled, the Linux kernel will reroute the valid interrupt to its legacy 114interrupt. This redirection of the handler will prevent the occurrence of 115the spurious interrupt detection which would ordinarily disable the IRQ 116line due to excessive unhandled counts. [2]_ 117 118The config option X86_REROUTE_FOR_BROKEN_BOOT_IRQS exists to enable (or 119disable) the redirection of the interrupt handler to the PCH interrupt 120line. The option can be overridden by either pci=ioapicreroute or 121pci=noioapicreroute. [3]_ 122 123 124More Documentation 125================== 126 127There is an overview of the legacy interrupt handling in several datasheets 128(6300ESB and 6700PXH below). While largely the same, it provides insight 129into the evolution of its handling with chipsets. 130 131Example of disabling of the boot interrupt 132------------------------------------------ 133 134 - Intel® 6300ESB I/O Controller Hub (Document # 300641-004US) 135 5.7.3 Boot Interrupt 136 https://www.intel.com/content/dam/doc/datasheet/6300esb-io-controller-hub-datasheet.pdf 137 138 - Intel® Xeon® Processor E5-1600/2400/2600/4600 v3 Product Families 139 Datasheet - Volume 2: Registers (Document # 330784-003) 140 6.6.41 cipintrc Coherent Interface Protocol Interrupt Control 141 https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/xeon-e5-v3-datasheet-vol-2.pdf 142 143Example of handler rerouting 144---------------------------- 145 146 - Intel® 6700PXH 64-bit PCI Hub (Document # 302628) 147 2.15.2 PCI Express Legacy INTx Support and Boot Interrupt 148 https://www.intel.com/content/dam/doc/datasheet/6700pxh-64-bit-pci-hub-datasheet.pdf 149 150 151If you have any legacy PCI interrupt questions that aren't answered, email me. 152 153Cheers, 154 Sean V Kelley 155 sean.v.kelley@linux.intel.com 156 157.. [1] https://lore.kernel.org/r/12131949181903-git-send-email-sassmann@suse.de/ 158.. [2] https://lore.kernel.org/r/12131949182094-git-send-email-sassmann@suse.de/ 159.. [3] https://lore.kernel.org/r/487C8EA7.6020205@suse.de/ 160