Fix incorrect MNNVL fabric check

[nvcastet]

Description

Fix incorrect MNNVL fabric check

Fixes # (issue)

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• 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)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Change A
• Change B

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[nvcastet]

@ptrendx Can you trigger CI?

[greptile-apps]

Greptile Overview

Greptile Summary

Fixed the MNNVL (Multi-Node NVLink) fabric detection logic in the has_mnnvl_fabric function. The previous implementation incorrectly treated the clusterUuid byte array as a null-terminated C-string by only checking the first byte, which could lead to false positives. The fix properly compares the entire UUID byte array using memcmp against a zero-initialized array.

Key changes:

• Removed manual null-terminator initialization of fabricInfo.clusterUuid[0]
• Changed state check from >= to == for NVML_GPU_FABRIC_STATE_COMPLETED (more precise matching)
• Replaced single-byte check with proper memcmp comparison across the entire NVML_GPU_FABRIC_UUID_LEN array

Confidence Score: 5/5

• This PR is safe to merge with minimal risk - it fixes a clear bug in UUID comparison logic
• The change corrects a clear logic error where a byte array was being treated as a C-string. The fix is straightforward, well-scoped, and follows proper byte array comparison practices. No breaking changes or side effects expected.
• No files require special attention

Important Files Changed

Filename	Overview
transformer_engine/common/comm_gemm_overlap/userbuffers/userbuffers-host.cpp	Fixed MNNVL fabric UUID check to properly compare byte array instead of treating it as C-string, and corrected state comparison to exact match

Sequence Diagram

sequenceDiagram
    participant Caller
    participant has_mnnvl_fabric
    participant CUDA_Driver
    participant NVML

    Caller->>has_mnnvl_fabric: Check MNNVL fabric support(device_id)
    has_mnnvl_fabric->>CUDA_Driver: cuDeviceGet(&dev, device_id)
    CUDA_Driver-->>has_mnnvl_fabric: device handle
    has_mnnvl_fabric->>CUDA_Driver: cuDeviceGetAttribute(FABRIC_SUPPORTED)
    CUDA_Driver-->>has_mnnvl_fabric: fabric_handle_supported
    
    alt fabric_handle_supported
        has_mnnvl_fabric->>NVML: nvmlInit_v2()
        has_mnnvl_fabric->>NVML: nvmlDeviceGetHandleByIndex_v2(device_id)
        NVML-->>has_mnnvl_fabric: local_device
        has_mnnvl_fabric->>has_mnnvl_fabric: Initialize fabricInfo
        has_mnnvl_fabric->>NVML: nvmlDeviceGetGpuFabricInfoV(local_device, &fabricInfo)
        NVML-->>has_mnnvl_fabric: fabricInfo with state and clusterUuid
        has_mnnvl_fabric->>NVML: nvmlShutdown()
        has_mnnvl_fabric->>has_mnnvl_fabric: Create zero_uuid[NVML_GPU_FABRIC_UUID_LEN]
        has_mnnvl_fabric->>has_mnnvl_fabric: Check state == COMPLETED && memcmp(clusterUuid, zero_uuid) != 0
        has_mnnvl_fabric-->>Caller: mnnvl_fabric_support
    else not supported
        has_mnnvl_fabric-->>Caller: false
    end

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[greptile-apps]

_{No files reviewed, no comments}

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[timmoon10]

LGTM, pending CI

[timmoon10]

/te-ci L1