The MSI Driver Guide HOWTO Tom L Nguyen tom.l.nguyen@intel.com 10/03/2003 Revised Feb 12, 2004 by Martine Silbermann email: Martine.Silbermann@hp.com Revised Jun 25, 2004 by Tom L Nguyen 1. About this guide This guide describes the basics of Message Signaled Interrupts (MSI), the advantages of using MSI over traditional interrupt mechanisms, and how to enable your driver to use MSI or MSI-X. Also included is a Frequently Asked Questions (FAQ) section. 1.1 Terminology PCI devices can be single-function or multi-function. In either case, when this text talks about enabling or disabling MSI on a "device function," it is referring to one specific PCI device and function and not to all functions on a PCI device (unless the PCI device has only one function). 2. Copyright 2003 Intel Corporation 3. What is MSI/MSI-X? Message Signaled Interrupt (MSI), as described in the PCI Local Bus Specification Revision 2.3 or later, is an optional feature, and a required feature for PCI Express devices. MSI enables a device function to request service by sending an Inbound Memory Write on its PCI bus to the FSB as a Message Signal Interrupt transaction. Because MSI is generated in the form of a Memory Write, all transaction conditions, such as a Retry, Master-Abort, Target-Abort or normal completion, are supported. A PCI device that supports MSI must also support pin IRQ assertion interrupt mechanism to provide backward compatibility for systems that do not support MSI. In systems which support MSI, the bus driver is responsible for initializing the message address and message data of the device function's MSI/MSI-X capability structure during device initial configuration. An MSI capable device function indicates MSI support by implementing the MSI/MSI-X capability structure in its PCI capability list. The device function may implement both the MSI capability structure and the MSI-X capability structure; however, the bus driver should not enable both. The MSI capability structure contains Message Control register, Message Address register and Message Data register. These registers provide the bus driver control over MSI. The Message Control register indicates the MSI capability supported by the device. The Message Address register specifies the target address and the Message Data register specifies the characteristics of the message. To request service, the device function writes the content of the Message Data register to the target address. The device and its software driver are prohibited from writing to these registers. The MSI-X capability structure is an optional extension to MSI. It uses an independent and separate capability structure. There are some key advantages to implementing the MSI-X capability structure over the MSI capability structure as described below. - Support a larger maximum number of vectors per function. - Provide the ability for system software to configure each vector with an independent message address and message data, specified by a table that resides in Memory Space. - MSI and MSI-X both support per-vector masking. Per-vector masking is an optional extension of MSI but a required feature for MSI-X. Per-vector masking provides the kernel the ability to mask/unmask a single MSI while running its interrupt service routine. If per-vector masking is not supported, then the device driver should provide the hardware/software synchronization to ensure that the device generates MSI when the driver wants it to do so. 4. Why use MSI? As a benefit to the simplification of board design, MSI allows board designers to remove out-of-band interrupt routing. MSI is another step towards a legacy-free environment. Due to increasing pressure on chipset and processor packages to reduce pin count, the need for interrupt pins is expected to diminish over time. Devices, due to pin constraints, may implement messages to increase performance. PCI Express endpoints uses INTx emulation (in-band messages) instead of IRQ pin assertion. Using INTx emulation requires interrupt sharing among devices connected to the same node (PCI bridge) while MSI is unique (non-shared) and does not require BIOS configuration support. As a result, the PCI Express technology requires MSI support for better interrupt performance. Using MSI enables the device functions to support two or more vectors, which can be configured to target different CPUs to increase scalability. 5. Configuring a driver to use MSI/MSI-X By default, the kernel will not enable MSI/MSI-X on all devices that support this capability. The CONFIG_PCI_MSI kernel option must be selected to enable MSI/MSI-X support. 5.1 Including MSI/MSI-X support into the kernel To allow MSI/MSI-X capable device drivers to selectively enable MSI/MSI-X (using pci_enable_msi()/pci_enable_msix() as described below), the VECTOR based scheme needs to be enabled by setting CONFIG_PCI_MSI during kernel config. Since the target of the inbound message is the local APIC, providing CONFIG_X86_LOCAL_APIC must be enabled as well as CONFIG_PCI_MSI. 5.2 Configuring for MSI support Due to the non-contiguous fashion in vector assignment of the existing Linux kernel, this version does not support multiple messages regardless of a device function is capable of supporting more than one vector. To enable MSI on a device function's MSI capability structure requires a device driver to call the function pci_enable_msi() explicitly. 5.2.1 API pci_enable_msi int pci_enable_msi(struct pci_dev *dev) With this new API, a device driver that wants to have MSI enabled on its device function must call this API to enable MSI. A successful call will initialize the MSI capability structure with ONE vector, regardless of whether a device function is capable of supporting multiple messages. This vector replaces the pre-assigned dev->irq with a new MSI vector. To avoid a conflict of the new assigned vector with existing pre-assigned vector requires a device driver to call this API before calling request_irq(). 5.2.2 API pci_disable_msi void pci_disable_msi(struct pci_dev *dev) This API should always be used to undo the effect of pci_enable_msi() when a device driver is unloading. This API restores dev->irq with the pre-assigned IOAPIC vector and switches a device's interrupt mode to PCI pin-irq assertion/INTx emulation mode. Note that a device driver should always call free_irq() on the MSI vector that it has done request_irq() on before calling this API. Failure to do so results in a BUG_ON() and a device will be left with MSI enabled and leaks its vector. 5.2.3 MSI mode vs. legacy mode diagram The below diagram shows the events which switch the interrupt mode on the MSI-capable device function between MSI mode and PIN-IRQ assertion mode. ------------ pci_enable_msi ------------------------ | | <=============== | | | MSI MODE | | PIN-IRQ ASSERTION MODE | | | ===============> | | ------------ pci_disable_msi ------------------------ Figure 1. MSI Mode vs. Legacy Mode In Figure 1, a device operates by default in legacy mode. Legacy in this context means PCI pin-irq assertion or PCI-Express INTx emulation. A successful MSI request (using pci_enable_msi()) switches a device's interrupt mode to MSI mode. A pre-assigned IOAPIC vector stored in dev->irq will be saved by the PCI subsystem and a new assigned MSI vector will replace dev->irq. To return back to its default mode, a device driver should always call pci_disable_msi() to undo the effect of pci_enable_msi(). Note that a device driver should always call free_irq() on the MSI vector it has done request_irq() on before calling pci_disable_msi(). Failure to do so results in a BUG_ON() and a device will be left with MSI enabled and leaks its vector. Otherwise, the PCI subsystem restores a device's dev->irq with a pre-assigned IOAPIC vector and marks the released MSI vector as unused. Once being marked as unused, there is no guarantee that the PCI subsystem will reserve this MSI vector for a device. Depending on the availability of current PCI vector resources and the number of MSI/MSI-X requests from other drivers, this MSI may be re-assigned. For the case where the PCI subsystem re-assigns this MSI vector to another driver, a request to switch back to MSI mode may result in being assigned a different MSI vector or a failure if no more vectors are available. 5.3 Configuring for MSI-X support Due to the ability of the system software to configure each vector of the MSI-X capability structure with an independent message address and message data, the non-contiguous fashion in vector assignment of the existing Linux kernel has no impact on supporting multiple messages on an MSI-X capable device functions. To enable MSI-X on a device function's MSI-X capability structure requires its device driver to call the function pci_enable_msix() explicitly. The function pci_enable_msix(), once invoked, enables either all or nothing, depending on the current availability of PCI vector resources. If the PCI vector resources are available for the number of vectors requested by a device driver, this function will configure the MSI-X table of the MSI-X capability structure of a device with requested messages. To emphasize this reason, for example, a device may be capable for supporting the maximum of 32 vectors while its software driver usually may request 4 vectors. It is recommended that the device driver should call this function once during the initialization phase of the device driver. Unlike the function pci_enable_msi(), the function pci_enable_msix() does not replace the pre-assigned IOAPIC dev->irq with a new MSI vector because the PCI subsystem writes the 1:1 vector-to-entry mapping into the field vector of each element contained in a second argument. Note that the pre-assigned IOAPIC dev->irq is valid only if the device operates in PIN-IRQ assertion mode. In MSI-X mode, any attempt at using dev->irq by the device driver to request for interrupt service may result unpredictabe behavior. For each MSI-X vector granted, a device driver is responsible for calling other functions like request_irq(), enable_irq(), etc. to enable this vector with its corresponding interrupt service handler. It is a device driver's choice to assign all vectors with the same interrupt service handler or each vector with a unique interrupt service handler. 5.3.1 Handling MMIO address space of MSI-X Table The PCI 3.0 specification has implementation notes that MMIO address space for a device's MSI-X structure should be isolated so that the software system can set different pages for controlling accesses to the MSI-X structure. The implementation of MSI support requires the PCI subsystem, not a device driver, to maintain full control of the MSI-X table/MSI-X PBA (Pending Bit Array) and MMIO address space of the MSI-X table/MSI-X PBA. A device driver is prohibited from requesting the MMIO address space of the MSI-X table/MSI-X PBA. Otherwise, the PCI subsystem will fail enabling MSI-X on its hardware device when it calls the function pci_enable_msix(). 5.3.2 Handling MSI-X allocation Determining the number of MSI-X vectors allocated to a function is dependent on the number of MSI capable devices and MSI-X capable devices populated in the system. The policy of allocating MSI-X vectors to a function is defined as the following: #of MSI-X vectors allocated to a function = (x - y)/z where x = The number of available PCI vector resources by the time the device driver calls pci_enable_msix(). The PCI vector resources is the sum of the number of unassigned vectors (new) and the number of released vectors when any MSI/MSI-X device driver switches its hardware device back to a legacy mode or is hot-removed. The number of unassigned vectors may exclude some vectors reserved, as defined in parameter NR_HP_RESERVED_VECTORS, for the case where the system is capable of supporting hot-add/hot-remove operations. Users may change the value defined in NR_HR_RESERVED_VECTORS to meet their specific needs. y = The number of MSI capable devices populated in the system. This policy ensures that each MSI capable device has its vector reserved to avoid the case where some MSI-X capable drivers may attempt to claim all available vector resources. z = The number of MSI-X capable devices populated in the system. This policy ensures that maximum (x - y) is distributed evenly among MSI-X capable devices. Note that the PCI subsystem scans y and z during a bus enumeration. When the PCI subsystem completes configuring MSI/MSI-X capability structure of a device as requested by its device driver, y/z is decremented accordingly. 5.3.3 Handling MSI-X shortages For the case where fewer MSI-X vectors are allocated to a function than requested, the function pci_enable_msix() will return the maximum number of MSI-X vectors available to the caller. A device driver may re-send its request with fewer or equal vectors indicated in the return. For example, if a device driver requests 5 vectors, but the number of available vectors is 3 vectors, a value of 3 will be returned as a result of pci_enable_msix() call. A function could be designed for its driver to use only 3 MSI-X table entries as different combinations as ABC--, A-B-C, A--CB, etc. Note that this patch does not support multiple entries with the same vector. Such attempt by a device driver to use 5 MSI-X table entries with 3 vectors as ABBCC, AABCC, BCCBA, etc will result as a failure by the function pci_enable_msix(). Below are the reasons why supporting multiple entries with the same vector is an undesirable solution. - The PCI subsystem cannot determine the entry that generated the message to mask/unmask MSI while handling software driver ISR. Attempting to walk through all MSI-X table entries (2048 max) to mask/unmask any match vector is an undesirable solution. - Walking through all MSI-X table entries (2048 max) to handle SMP affinity of any match vector is an undesirable solution. 5.3.4 API pci_enable_msix int pci_enable_msix(struct pci_dev *dev, struct msix_entry *entries, int nvec) This API enables a device driver to request the PCI subsystem to enable MSI-X messages on its hardware device. Depending on the availability of PCI vectors resources, the PCI subsystem enables either all or none of the requested vectors. Argument 'dev' points to the device (pci_dev) structure. Argument 'entries' is a pointer to an array of msix_entry structs. The number of entries is indicated in argument 'nvec'. struct msix_entry is defined in /driver/pci/msi.h: struct msix_entry { u16 vector; /* kernel uses to write alloc vector */ u16 entry; /* driver uses to specify entry */ }; A device driver is responsible for initializing the field 'entry' of each element with a unique entry supported by MSI-X table. Otherwise, -EINVAL will be returned as a result. A successful return of zero indicates the PCI subsystem completed initializing each of the requested entries of the MSI-X table with message address and message data. Last but not least, the PCI subsystem will write the 1:1 vector-to-entry mapping into the field 'vector' of each element. A device driver is responsible for keeping track of allocated MSI-X vectors in its internal data structure. A return of zero indicates that the number of MSI-X vectors was successfully allocated. A return of greater than zero indicates MSI-X vector shortage. Or a return of less than zero indicates a failure. This failure may be a result of duplicate entries specified in second argument, or a result of no available vector, or a result of failing to initialize MSI-X table entries. 5.3.5 API pci_disable_msix void pci_disable_msix(struct pci_dev *dev) This API should always be used to undo the effect of pci_enable_msix() when a device driver is unloading. Note that a device driver should always call free_irq() on all MSI-X vectors it has done request_irq() on before calling this API. Failure to do so results in a BUG_ON() and a device will be left with MSI-X enabled and leaks its vectors. 5.3.6 MSI-X mode vs. legacy mode diagram The below diagram shows the events which switch the interrupt mode on the MSI-X capable device function between MSI-X mode and PIN-IRQ assertion mode (legacy). ------------ pci_enable_msix(,,n) ------------------------ | | <=============== | | | MSI-X MODE | | PIN-IRQ ASSERTION MODE | | | ===============> | | ------------ pci_disable_msix ------------------------ Figure 2. MSI-X Mode vs. Legacy Mode In Figure 2, a device operates by default in legacy mode. A successful MSI-X request (using pci_enable_msix()) switches a device's interrupt mode to MSI-X mode. A pre-assigned IOAPIC vector stored in dev->irq will be saved by the PCI subsystem; however, unlike MSI mode, the PCI subsystem will not replace dev->irq with assigned MSI-X vector because the PCI subsystem already writes the 1:1 vector-to-entry mapping into the field 'vector' of each element specified in second argument. To return back to its default mode, a device driver should always call pci_disable_msix() to undo the effect of pci_enable_msix(). Note that a device driver should always call free_irq() on all MSI-X vectors it has done request_irq() on before calling pci_disable_msix(). Failure to do so results in a BUG_ON() and a device will be left with MSI-X enabled and leaks its vectors. Otherwise, the PCI subsystem switches a device function's interrupt mode from MSI-X mode to legacy mode and marks all allocated MSI-X vectors as unused. Once being marked as unused, there is no guarantee that the PCI subsystem will reserve these MSI-X vectors for a device. Depending on the availability of current PCI vector resources and the number of MSI/MSI-X requests from other drivers, these MSI-X vectors may be re-assigned. For the case where the PCI subsystem re-assigned these MSI-X vectors to other drivers, a request to switch back to MSI-X mode may result being assigned with another set of MSI-X vectors or a failure if no more vectors are available. 5.4 Handling function implementing both MSI and MSI-X capabilities For the case where a function implements both MSI and MSI-X capabilities, the PCI subsystem enables a device to run either in MSI mode or MSI-X mode but not both. A device driver determines whether it wants MSI or MSI-X enabled on its hardware device. Once a device driver requests for MSI, for example, it is prohibited from requesting MSI-X; in other words, a device driver is not permitted to ping-pong between MSI mod MSI-X mode during a run-time. 5.5 Hardware requirements for MSI/MSI-X support MSI/MSI-X support requires support from both system hardware and individual hardware device functions. 5.5.1 System hardware support Since the target of MSI address is the local APIC CPU, enabling MSI/MSI-X support in the Linux kernel is dependent on whether existing system hardware supports local APIC. Users should verify that their system supports local APIC operation by testing that it runs when CONFIG_X86_LOCAL_APIC=y. In SMP environment, CONFIG_X86_LOCAL_APIC is automatically set; however, in UP environment, users must manually set CONFIG_X86_LOCAL_APIC. Once CONFIG_X86_LOCAL_APIC=y, setting CONFIG_PCI_MSI enables the VECTOR based scheme and the option for MSI-capable device drivers to selectively enable MSI/MSI-X. Note that CONFIG_X86_IO_APIC setting is irrelevant because MSI/MSI-X vector is allocated new during runtime and MSI/MSI-X support does not depend on BIOS support. This key independency enables MSI/MSI-X support on future IOxAPIC free platforms. 5.5.2 Device hardware support The hardware device function supports MSI by indicating the MSI/MSI-X capability structure on its PCI capability list. By default, this capability structure will not be initialized by the kernel to enable MSI during the system boot. In other words, the device function is running on its default pin assertion mode. Note that in many cases the hardware supporting MSI have bugs, which may result in system hangs. The software driver of specific MSI-capable hardware is responsible for deciding whether to call pci_enable_msi or not. A return of zero indicates the kernel successfully initialized the MSI/MSI-X capability structure of the device function. The device function is now running on MSI/MSI-X mode. 5.6 How to tell whether MSI/MSI-X is enabled on device function At the driver level, a return of zero from the function call of pci_enable_msi()/pci_enable_msix() indicates to a device driver that its device function is initialized successfully and ready to run in MSI/MSI-X mode. At the user level, users can use the command 'cat /proc/interrupts' to display the vectors allocated for devices and their interrupt MSI/MSI-X modes ("PCI-MSI"/"PCI-MSI-X"). Below shows MSI mode is enabled on a SCSI Adaptec 39320D Ultra320 controller. CPU0 CPU1 0: 324639 0 IO-APIC-edge timer 1: 1186 0 IO-APIC-edge i8042 2: 0 0 XT-PIC cascade 12: 2797 0 IO-APIC-edge i8042 14: 6543 0 IO-APIC-edge ide0 15: 1 0 IO-APIC-edge ide1 169: 0 0 IO-APIC-level uhci-hcd 185: 0 0 IO-APIC-level uhci-hcd 193: 138 10 PCI-MSI aic79xx 201: 30 0 PCI-MSI aic79xx 225: 30 0 IO-APIC-level aic7xxx 233: 30 0 IO-APIC-level aic7xxx NMI: 0 0 LOC: 324553 325068 ERR: 0 MIS: 0 6. FAQ Q1. Are there any limitations on using the MSI? A1. If the PCI device supports MSI and conforms to the specification and the platform supports the APIC local bus, then using MSI should work. Q2. Will it work on all the Pentium processors (P3, P4, Xeon, AMD processors)? In P3 IPI's are transmitted on the APIC local bus and in P4 and Xeon they are transmitted on the system bus. Are there any implications with this? A2. MSI support enables a PCI device sending an inbound memory write (0xfeexxxxx as target address) on its PCI bus directly to the FSB. Since the message address has a redirection hint bit cleared, it should work. Q3. The target address 0xfeexxxxx will be translated by the Host Bridge into an interrupt message. Are there any limitations on the chipsets such as Intel 8xx, Intel e7xxx, or VIA? A3. If these chipsets support an inbound memory write with target address set as 0xfeexxxxx, as conformed to PCI specification 2.3 or latest, then it should work. Q4. From the driver point of view, if the MSI is lost because of errors occurring during inbound memory write, then it may wait forever. Is there a mechanism for it to recover? A4. Since the target of the transaction is an inbound memory write, all transaction termination conditions (Retry, Master-Abort, Target-Abort, or normal completion) are supported. A device sending an MSI must abide by all the PCI rules and conditions regarding that inbound memory write. So, if a retry is signaled it must retry, etc... We believe that the recommendation for Abort is also a retry (refer to PCI specification 2.3 or latest).