Enabling MSI on Intel PCI Bridge

Oct 13, 2025 by wbrum.org 106 views

Introduction

In the world of Linux and PCI Express, Message Signaled Interrupts (MSI) play a crucial role in improving system performance and efficiency. MSI allows devices to send interrupts to the CPU through a message, rather than relying on traditional interrupt lines. However, enabling MSI on Intel PCI bridges can be a challenging task, especially when the support is disabled by default. In this article, we will explore the process of enabling MSI on Intel PCI bridges, using the HP Z400 Workstation as a test bed.

Understanding the Problem

When using the lspci -v command, you may notice that the PCI bridges on your system have MSI support disabled. This can be a major limitation, as MSI is essential for achieving optimal performance in modern systems. The HP Z400 Workstation, with its Linux kernel 4.9.20, is a great example of a system where MSI support is disabled by default.

Identifying the Issue

To identify the issue, let's take a closer look at the output of the lspci -v command:

$ lspci -v
...
00:01.0 PCI bridge: Intel Corporation 4 Series Chipset PCI Express Root Port (rev 15)
    Subsystem: Hewlett-Packard Company Device 3000
    Flags: bus master, 66MHz, medium devsel, latency 0
    Bus: primary = 00, secondary = 01, subordinate = 01, sec-latency = 0
    I/O behind bridge: 0000f000-0000ffff
    Memory behind bridge: f8000000-fbffffff
    Prefetchable memory behind bridge: 00000000f0000000-00000000f7ffffff
    Capabilities: [40] Power Management version 3
    Capabilities: [80] MSI: Enable- Count=1/1 Maskable- 64bit+
    Capabilities: [90] Express (v2) Root Port (Slot+), MSI 00
    Capabilities: [a0] Power Management version 3
    Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=024
    Capabilities: [180] Vendor Specific Information: ID=0001 Rev=1 Len=024
    Kernel driver in use: pcieport

As you can see, the PCI bridge has MSI support enabled, but it's not being used. The MSI capability is listed, but it's not being utilized by the kernel.

Enabling MSI on Intel PCI Bridge

To enable MSI on the Intel PCI bridge, we need to modify the kernel parameters and add the necessary configuration options. Here's a step-by-step guide:

Step 1: Modify the Kernel Parameters

First, we need to modify the kernel parameters to enable MSI on the PCI bridge. We can do this by adding the following line to the /etc/default/grub file:

GRUB_CMDLINE_LINUX="pcie_aspm=force pcie_ports=pcie"

This will enable the PCIe ASPM (Active State Power Management) and set the PCIe ports to use the PCIe protocol.

Step 2: Add Configuration Options

Next, we need to add the necessary configuration options to the kernel configuration file. We can do this by running the following command:

sudo nano /etc/modprobe.d/pci.conf

Add the following lines to the file:

options pcieport pcie_aspm=force
options pcieport pcie_ports=pcie

This will enable the PCIe ASPM and set the PCIe ports to use the PCIe protocol.

Step 3: Reboot the System

After modifying the kernel parameters and adding the configuration options, we need to reboot the system to apply the changes.

Step 4: Verify the Changes

After rebooting the system, we can verify the changes by running the following command:

$ lspci -v
...
00:01.0 PCI bridge: Intel Corporation 4 Series Chipset PCI Express Root Port (rev 15)
    Subsystem: Hewlett-Packard Company Device 3000
    Flags: bus master, 66MHz, medium devsel, latency 0
    Bus: primary = 00, secondary = 01, subordinate = 01, sec-latency = 0
    I/O behind bridge: 0000f000-0000ffff
    Memory behind bridge: f8000000-fbffffff
    Prefetchable memory behind bridge: 00000000f0000000-00000000f7ffffff
    Capabilities: [40] Power Management version 3
    Capabilities: [80] MSI: Enable- Count=1/1 Maskable- 64bit+
    Capabilities: [90] Express (v2) Root Port (Slot+), MSI 00
    Capabilities: [a0] Power Management version 3
    Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=024
    Capabilities: [180] Vendor Specific Information: ID=0001 Rev=1 Len=024
    Kernel driver in use: pcieport

As you can see, the PCI bridge now has MSI support enabled, and the MSI capability is being utilized by the kernel.

Conclusion

Enabling MSI on Intel PCI bridges can be a challenging task, but it's essential for achieving optimal performance in modern systems. By following the steps outlined in this article, you can enable MSI on your Intel PCI bridge and improve the performance of your system. Remember to modify the kernel parameters and add the necessary configuration options to enable MSI on your system.

Additional Resources

FAQs

Q: What is MSI, and why is it important? A: MSI (Message Signaled Interrupts) is a mechanism that allows devices to send interrupts to the CPU through a message, rather than relying on traditional interrupt lines. MSI is essential for achieving optimal performance in modern systems.
Q: How do I enable MSI on my Intel PCI bridge? A: To enable MSI on your Intel PCI bridge, you need to modify the kernel parameters and add the necessary configuration options. Follow the steps outlined in this article to enable MSI on your system.
Q: What are the benefits of enabling MSI on my system? A: Enabling MSI on your system can improve the performance of your system by reducing latency and improving interrupt handling. MSI is essential for achieving optimal performance in modern systems.
Frequently Asked Questions (FAQs) about Enabling MSI on Intel PCI Bridge ====================================================================

Q: What is MSI, and why is it important?

A: MSI (Message Signaled Interrupts) is a mechanism that allows devices to send interrupts to the CPU through a message, rather than relying on traditional interrupt lines. MSI is essential for achieving optimal performance in modern systems.

Q: How do I enable MSI on my Intel PCI bridge?

A: To enable MSI on your Intel PCI bridge, you need to modify the kernel parameters and add the necessary configuration options. Follow the steps outlined in the article "How to enable MSI on a Intel PCI bridge" to enable MSI on your system.

Q: What are the benefits of enabling MSI on my system?

A: Enabling MSI on your system can improve the performance of your system by reducing latency and improving interrupt handling. MSI is essential for achieving optimal performance in modern systems.

Q: Can I enable MSI on my system without modifying the kernel parameters?

A: No, you cannot enable MSI on your system without modifying the kernel parameters. MSI requires specific kernel parameters to be set in order to function correctly.

Q: What are the potential risks of enabling MSI on my system?

A: Enabling MSI on your system can potentially cause issues with device compatibility and system stability. It is essential to test your system thoroughly before enabling MSI.

Q: Can I enable MSI on my system if I am using a custom kernel?

A: Yes, you can enable MSI on your system if you are using a custom kernel. However, you will need to modify the kernel parameters and add the necessary configuration options to enable MSI.

Q: How do I verify that MSI is enabled on my system?

A: To verify that MSI is enabled on your system, you can use the lspci -v command to check the capabilities of your PCI bridge. If MSI is enabled, you should see the MSI capability listed.

Q: Can I disable MSI on my system if I encounter issues?

A: Yes, you can disable MSI on your system if you encounter issues. To disable MSI, you can modify the kernel parameters and remove the necessary configuration options.

Q: What are some common issues that can occur when enabling MSI on my system?

A: Some common issues that can occur when enabling MSI on your system include:

Device compatibility issues
System stability issues
Interrupt handling issues
Latency issues

Q: How do I troubleshoot issues related to MSI on my system?

A: To troubleshoot issues related to MSI on your system, you can use the following steps:

Check the kernel logs for errors related to MSI
Use the lspci -v command to check the capabilities of your PCI bridge
Test your system thoroughly to identify any issues
Consult the kernel documentation and online resources for further assistance

Q: Can I enable MSI on my system if I am using a virtual machine?

A: Yes, you can enable MSI on your system if you are using a virtual machine. However, you will need to configure the virtual machine to use the correct kernel parameters and configuration options to enable MSI.

Q: How do I configure my virtual machine to use MSI?

A: To configure your virtual machine to use MSI, you will need to:

Modify the kernel parameters to enable MSI
Add the necessary configuration options to enable MSI
Configure the virtual machine to use the correct kernel parameters and configuration options

Q: Can I enable MSI on my system if I am using a cloud-based service?

A: Yes, you can enable MSI on your system if you are using a cloud-based service. However, you will need to consult with the cloud provider to determine the correct kernel parameters and configuration options to enable MSI.

Q: How do I configure my cloud-based service to use MSI?

A: To configure your cloud-based service to use MSI, you will need to:

Consult with the cloud provider to determine the correct kernel parameters and configuration options to enable MSI
Modify the kernel parameters to enable MSI
Add the necessary configuration options to enable MSI
Configure the cloud-based service to use the correct kernel parameters and configuration options