Linux kernel CPUID detection

CPUID (CPU Identification) is a CPU identification and feature detection mechanism supported by modern CPUs. It allows software to query the CPU about its supported features and capabilities, such as the number of cores, the cache size, and the supported instruction set.

CPUID is implemented in hardware, and it is accessed through a special instruction that is executed by the CPU. The instruction takes an input value, which specifies the type of information that is being requested, and returns an output value, which contains the requested information.

The Linux kernel provides a number of interfaces for accessing CPUID information. These interfaces are defined in the /usr/include/asm/cpufeature.h header file.

The most commonly used interface is the cpuid() function. This function takes an input value, which specifies the type of information that is being requested, and returns an output value, which contains the requested information.

The following table shows some of the most commonly used CPUID input values:

Input Value	Description
0	Get the basic CPU information, such as the CPU vendor, the CPU family, and the CPU model.
1	Get the feature flags for the CPU. These flags indicate which features are supported by the CPU.
7	Get the extended feature flags for the CPU. These flags indicate which extended features are supported by the CPU.

The output value from the cpuid() function is a 32-bit integer. The high 32 bits of the output value contain the requested information. The low 32 bits of the output value are reserved for future use.

The following code sample shows how to use the cpuid() function to get the basic CPU information:

#include <stdio.h>
#include <asm/cpufeature.h>

int main() {
  unsigned int eax, ebx, ecx, edx;

  cpuid(0, &eax, &ebx, &ecx, &edx);

  printf("CPU vendor: %sn", (char *)&ebx);
  printf("CPU family: %un", (eax >> 8) & 0xf);
  printf("CPU model: %un", (eax >> 4) & 0xf);

  return 0;
}

The output of the code sample will look something like this:

CPU vendor: GenuineIntel
CPU family: 6
CPU model: 15

CPUID and Linux kernel development

CPUID information is used by the Linux kernel to determine the capabilities of the CPU and to configure the kernel accordingly. For example, the kernel uses CPUID information to:

• Determine the number of cores in the CPU and to distribute tasks across the cores.
• Enable or disable certain features, such as hardware virtualization and power management.
• Optimize the kernel for the specific CPU architecture.

CPUID information is also used by a number of user-space applications, such as:

• System monitoring tools, such as lscpu and htop.
• Benchmarking tools, such as phoronix-test-suite and Geekbench.
• Virtualization software, such as VirtualBox and QEMU.

Frequently Asked Questions (FAQ)

What is CPUID?

CPUID is a CPU identification and feature detection mechanism supported by modern CPUs. It allows software to query the CPU about its supported features and capabilities.

How do I access CPUID information in the Linux kernel?

The Linux kernel provides a number of interfaces for accessing CPUID information. The most commonly used interface is the cpuid() function.

What is the difference between CPUID and ACPI?

CPUID is a CPU identification and feature detection mechanism, while ACPI is a power management and configuration interface. CPUID is implemented in hardware, while ACPI is implemented in software.

How can I use CPUID information to optimize my Linux kernel?

You can use CPUID information to determine the capabilities of your CPU and to configure the Linux kernel accordingly. For example, you can use CPUID information to:

• Determine the number of cores in the CPU and to distribute tasks across the cores.
• Enable or disable certain features, such as hardware virtualization and power management.
• Optimize the kernel for the specific CPU architecture.

References

• Intel® 64 and IA-32 Architectures Software Developer’s Manual
• CPUID – Wikipedia
• The Linux Kernel CPUID Interface

Intel CPUID Flags for Linux

CPUID flags provide information about the features and capabilities of an Intel processor. In Linux, these flags can be accessed using the cpuid command. The output of the cpuid command is a hexadecimal string that contains a series of flags.

The following table lists some of the most important CPUID flags for Linux:

Flag	Description
fpu	Floating-point unit
mmx	MMX instructions
sse	SSE instructions
sse2	SSE2 instructions
avx	AVX instructions
avx2	AVX2 instructions

These flags can be used to determine which features are supported by a particular processor. This information can be useful for optimizing software performance. For example, if a program uses AVX instructions, it is important to ensure that the processor supports AVX before running the program.

To find out which CPUID flags are supported by your processor, run the following command:

$ cpuid | grep -E '^flags:'

The output of this command will be a hexadecimal string that contains a series of flags. The flags that are set to 1 are supported by your processor.

AMD Epyc CPUID Features

The AMD Epyc CPUID features provide information about the processor’s capabilities and characteristics. These features include:

• Processor Identification: Provides information about the processor’s vendor, model, and family.
• Feature Support: Indicates supported features such as virtual machine extensions, memory type, and power management capabilities.
• Extended Features: Includes additional information about processor features, such as AVX extensions, cache hierarchy, and processor topology.
• Brand String: Provides a descriptive string that identifies the processor brand.
• Processor Topology: Provides details about the number of cores, threads, and sockets in the processor.
• Cache Information: Provides information about the processor’s cache hierarchy, including the size and type of each cache.
• Thermal and Power Management: Provides information about the processor’s temperature, power consumption, and voltage requirements.

CPUID Support for KVM

KVM (Kernel-based Virtual Machine) provides CPUID (CPU Identification) support to enable virtual machines to accurately report CPU features and capabilities. CPUID is a processor instruction that retrieves information about the CPU’s architecture, features, and capabilities.

KVM supports CPUID features such as:

• Basic CPU identification (vendor ID, family, model, etc.)
• Advanced features (e.g., SSE, MMX, AVX)
• Topology information (e.g., number of cores, threads)

KVM achieves CPUID support by:

• Intercepting CPUID instructions from the guest VM
• Translating the instructions to the host CPU
• Returning appropriate CPUID values
• Providing additional hooks for adjusting the reported values

This CPUID support allows guest VMs to accurately identify and utilize the features of the host CPUs, ensuring optimal performance and compatibility. It also facilitates the migration of guest VMs between different host machines with different CPU configurations.

Troubleshooting CPUID Issues in Linux

When encountering CPUID issues in Linux, check the following:

• Ensure CPUID is supported: Use lscpu or cat /proc/cpuinfo to verify if CPUID is supported.
• Enable CPUID: In BIOS/UEFI settings, ensure CPUID support is enabled.
• Check software support: Ensure the Linux kernel and CPUID driver are updated.
• Isolate the issue: Run cpuid command and examine the output for any errors or missing features.
• Check for CPUID emulation: Some hypervisors may emulate CPUID, which can lead to discrepancies.
• Verify hardware configuration: Ensure the CPU is properly installed and configured.
• Update BIOS/UEFI firmware: Outdated firmware can cause CPUID issues.
• Check for hardware errors: Use dmesg or journalctl to check for hardware-related error messages.
• Report the issue: If unable to resolve the issue, report it to the Linux kernel or CPU manufacturer for further assistance.

Kernel-based Virtual Machine CPUID Passthrough

Kernel-based Virtual Machine (KVM) CPUID passthrough allows virtual machines (VMs) to access the host CPU’s CPUID information directly. This provides enhanced performance and compatibility for guest operating systems within VMs.

CPUID (Central Processor IDentification) is a processor identification feature that allows software to query the capabilities and features of the CPU. By passing through the CPUID information to the VM, it eliminates the need for emulation and provides a more accurate representation of the host CPU. This improves performance-intensive operations such as virtualization, data encryption, and multimedia processing.

CPUID passthrough requires a hardware-assisted virtualization (HV) feature called CPUID spoofing or leaf spoofing, which must be supported by the host system. This feature enables the VM to access the native CPUID information without interfering with the host operating system.

Linux Kernel CPUID Performance Optimization

The CPUID instruction is a critical instruction for performance optimization in the Linux kernel. It allows the kernel to query the processor for its features and capabilities. However, relying on traditional methods to handle CPUID can introduce significant performance overhead.

To address this, the Linux kernel has implemented various performance optimizations techniques:

• x86_cpuid_load_leaf(): This function is the primary mechanism for querying the CPUID instruction. It has been optimized to reduce the number of CPU cycles required to retrieve the requested leaf.
• CPUID leaf caching: The kernel caches the results of frequently used CPUID leaves to avoid the need for repeated queries.
• CPUID lazy evaluation: Thekernel defers the retrieval of certain CPUID leaves until they are actually required.
• CPUID batching: The kernel batches multiple CPUID queries into a single operation to minimize the number of round-trips to the processor.

These performance optimizations significantly reduce the overhead associated with CPUID usage, which can result in noticeable performance improvements for workloads that heavily rely on processor features identification and utilization.

Detailed Analysis of Linux Kernel CPUID

The CPUID instruction is a powerful tool for gathering information about the underlying processor. Linux kernel provides a detailed analysis of the CPUID data in the form of a sysfs file /proc/cpuinfo.

This file contains a wealth of information about the CPU’s:

• Vendor
• Model
• Clock Speed
• Cache Hierarchies
• Feature Flags
• Topology

The data provided by /proc/cpuinfo can be used for a variety of purposes, such as:

• Detecting and identifying specific CPU models
• Configuring the kernel to optimize performance for a particular CPU
• Gathering system information for diagnostic or profiling purposes

The CPUID analysis in the Linux kernel is constantly being improved to provide more detailed and accurate information about the latest processors. As a result, /proc/cpuinfo is a valuable resource for anyone who wants to learn more about their CPU or optimize their system’s performance.

Advanced Techniques for Using CPUID in Linux

CPUID Levels and Flags

CPUID can access different levels of information by passing specific input values. Level 0, for example, provides basic processor information, while higher levels reveal more detailed features. Flags such as cpuid_supported and cpuid_level indicate available levels.

Vendor-Specific Extensions

Certain vendors implement vendor-specific extensions to CPUID that are not standardized. To access these, use the leaf and subleaf arguments to specify the vendor and extension respectively.

Performance Optimization

To improve performance when using CPUID, consider using the -m cpuid flag with the perf command to generate CPUID instructions in assembly. Additionally, avoid using CPUID loops, as they can be inefficient.

Using CPUID in Kernel Modules

CPUID can be accessed from kernel modules using the cpuid function. This allows modules to query processor features and optimize code accordingly.

Advanced Analysis and Reporting

Tools like cpuid2html and cpuid2csv can be used to generate detailed reports on CPUID information. These reports can provide insights into processor capabilities, power consumption, and other aspects.

CPUID Implementation in Linux Kernels

CPUID (Central Processing Unit Identification) is an instruction used to gather information about the processor. Different Linux kernels implement CPUID in unique ways, which can be summarized as follows:

• Pre-2.6.16: CPUID was handled directly in the kernel code.
• 2.6.16-2.6.29: Introduced K8-style CPUID handling in x86_64, where CPUID values are cached in a per-CPU structure.
• 2.6.30-5.6: Further refined per-CPU caching and added support for NUMA architectures.
• 5.7-present: Introduced generic CPUID handling, moving CPUID caching to the perf subsystem and enabling access to CPUID information from user space.

This evolution has improved the efficiency and versatility of CPUID handling in Linux kernels, allowing for accurate information retrieval and adaptation to various architectural advancements.

CPUID and Its Role in Linux Performance Tuning

CPUID (CPU Identification) is a valuable tool in Linux performance tuning. It allows accessing information about the underlying CPU architecture and features, enabling optimizations for specific workloads. By leveraging CPUID data, system administrators and developers can:

• Identify CPU capabilities and limitations: CPUID provides detailed information on processor family, model, and features such as instruction sets, cache sizes, and virtualization support. This knowledge helps in optimizing code for specific CPUs.
• Optimize instruction selection: CPUs have different instruction sets and performance characteristics. CPUID can identify the available instructions, allowing developers to choose the most efficient ones for their code.
• Tune memory access: CPUID provides insights into memory architecture, such as cache size and associativity. This information helps in optimizing data structures and algorithms to minimize cache misses.
• Enable hardware acceleration: Many CPUs support hardware acceleration features, such as AVX and AES. CPUID can detect these features, allowing programs to leverage them for improved performance.

By leveraging CPUID data and understanding the underlying CPU architecture, system administrators and developers can effectively tune their Linux systems and applications for optimal performance.

Customizing CPUID Data in the Linux Kernel

CPUID (CPU Identification) data provides information about a system’s processor, allowing applications to optimize their performance accordingly. The Linux kernel allows customization of this data to provide specific details to users or protect sensitive information.

To customize CPUID data, the following methods can be employed:

• CPU ID Override Framework: This framework allows developers to register custom functions to modify or replace CPUID data returned to user-space.
• Software CPUID Functions: By implementing software CPUID functions, custom data can be injected into the CPUID stream.
• Model-Specific Registers: Specific registers can be used to control the behavior of CPUID reporting, such as the MSRs (Model-Specific Registers) in x86 processors.

Customizing CPUID data can be useful for:

• Security: Overriding sensitive data to prevent potential security vulnerabilities.
• Performance Analysis: Injecting custom data to facilitate performance optimization in applications.
• Emulation: Modifying CPUID data to create virtualized or emulated environments.

Exploring CPUID Capabilities for Virtualization in Linux

CPUID (Processor Identification and Information) instructions provide information about the processor’s features and capabilities. For virtualization, key CPU features include:

• VT-x (Intel Virtualization Technology): Support for hosting virtual machines in a hypervisor.
• VT-d (Intel Virtualization Technology for Directed I/O): Enables assignment of I/O devices to virtual machines.
• AMD-V (AMD Virtualization): AMD’s equivalent to VT-x.
• SVM (Secure Virtual Machine): AMD’s equivalent to VT-d.

To check for CPUID virtualization capabilities in Linux, use the lscpu command:

sudo lscpu | grep -i virtualization

Output examples:

• Intel VT-x and VT-d: Virtualization: VT-x, VT-d
• AMD-V and SVM: Virtualization: AMD-V, SVM

These flags indicate the presence of hardware virtualization capabilities, which can be further used by hypervisors to create and manage virtual machines.

Security Implications of CPUID Mishandling in Linux

CPU Identification (CPUID) is a critical instruction used in Linux to detect and configure hardware features. Mishandling CPUID can lead to security vulnerabilities:

• Privilege Escalation: Attackers can exploit mishandling to escalate privileges by triggering a kernel panic or crashing the system, allowing them to bypass security controls.
• Data Leakage: Mishandling can expose sensitive information through error messages, memory leaks, or debugging outputs, potentially compromising secrets or undermining encryption mechanisms.
• Denial-of-Service (DoS): Improper CPUID handling can lead to unexpected system crashes or hangs, rendering the system inaccessible or unusable.

To mitigate these risks, it is crucial to ensure proper CPUID handling in Linux kernel and software, including:

• Enforcing CPUID Restrictions: Limiting access to CPUID information based on user privileges and hardware security features.
• Validating CPUID Responses: Verifying the authenticity and integrity of CPUID responses to prevent forged or manipulated data.
• Secure Debugging: Restricting debug messages related to CPUID information and handling them securely to prevent information leakage.