Fix incorrect bitmask for MBM counter length

[shenxiaochen]

Description

From Intel RDT spec[1] and AMD Platform QoS spec[2]: If the CPU platform supports CPUID.0FH.01H:EAX, CPUID.0FH.01H:EAX[7:0] returns MBM counter length (width) as offset from 24.

But in hw_cap_mon_discover(), the MBM counter length is calculated with incorrect 7-bits bitmask (0x7f).

Fix the issue with 8-bits bitmask (0xff) for MBM counter length.

[1] Intel Architectures SDM, Vol.3B, 19.18 Intel RDT Monitoring: https://cdrdv2.intel.com/v1/dl/getContent/671200

[2] AMD Platform QoS Extensions, Rev 1.03:
https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/other/56375_1_03_PUB.pdf

Fixes: 050f8c6 ("lib: detect MBM counter length")

Affected parts

• library
• pqos utility
• rdtset utility
• other: (please specify)

Motivation and Context

Fix incorrect bitmask for MBM counter length

How Has This Been Tested?

1. Passed all tests in intel-cmt-cat/unit-test.
2. Run most pqos tests described below:
   https://github.com/intel/intel-cmt-cat/wiki/Usage-Examples

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[ajherdri]

Hi Xiaochen,
Thanks for noticing this! We will review and get back to you soon.
May take a week or so given the US holiday this week.
Thanks,
~Andrew

[ajherdri]

Update -- we have reviewed and plan to approve. We will merge this with the next RDT Utility release in Q1-2026, and we will include this fix as part of that, and the pull request will be marked as accepted at that point.
Thanks,
~Andrew

[Copilot]

Pull request overview

This PR fixes an incorrect bitmask used when calculating the MBM (Memory Bandwidth Monitoring) counter length from CPUID data. The issue stems from using a 7-bit mask (0x7f) instead of the correct 8-bit mask (0xff) as specified in both Intel RDT and AMD Platform QoS specifications.

• Corrects the bitmask from 0x7f to 0xff to properly extract bits [7:0] from CPUID.0FH.01H:EAX
• Aligns the implementation with Intel and AMD architectural specifications

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.