[HARDWARE][POWER] Enable SHM communicator support for PowerPC

[Rukhaiya2004]

Purpose

Enable SHM communicator support for PowerPC systems.

Test Plan

vllm serve ibm-granite/granite-3.3-8b-instruct --tensor-parallel-
size 2 --dtype bfloat16 --max-model-len 512 --port 8000

Test Result

Metric	Main Branch (Without SHM)	SHM Enabled Branch
Output Token Throughput (tok/s)	97.92	100.26
Total Token Throughput (tok/s)	195.85	200.52
Mean TTFT (ms)	27585.48	26814.95
Mean TPOT (ms)	806.69	789.12

[github-actions]

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[Akashcodes732]

Hi @bigPYJ1151 , can we please add the ready label to the PR so that tests can run ?

[mergify]

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[depthfirst-app]

⚪ Severity: LOW

Unlike the sibling BF16Vec16::save (which uses std::max(0, std::min(elem_num, 16))), FP16Vec16::save does not clamp elem_num to a non-negative value. A negative elem_num produces a negative num * 2 which, when implicitly converted to size_t for vec_xst_len, becomes a very large value, causing an unintended write of up to 16 bytes past the buffer boundary.
Helpful? Add 👍 / 👎

💡 Fix Suggestion

Suggestion: Clamp elem_num to a non-negative value to match the defensive pattern used by BF16Vec16::save. Replace std::min(elem_num, VEC_ELEM_NUM) with std::max(0, std::min(elem_num, VEC_ELEM_NUM)) so that a negative elem_num results in zero bytes written instead of an unintended large memcpy.

⚠️ Experimental Feature: This code suggestion is automatically generated. Please review carefully.

Suggested change

	int num = std::min(elem_num, VEC_ELEM_NUM);
	int num = std::max(0, std::min(elem_num, VEC_ELEM_NUM));

[Rukhaiya2004]

Hi @depthfirst-app , Thank you for catching this security issue!

I've applied the fix to clamp elem_num to non-negative values:

int num = std::max(0, std::min(elem_num, VEC_ELEM_NUM));

[Rukhaiya2004]

Hi @bigPYJ1151 , I've checked the failing tests after updating the branch. The failures are in tests/utils_/test_import_utils.py and target vllm/utils/import_utils.py, which appear unrelated to the SHM communicator changes in this PR. Could you please confirm if we can proceed with the merge?

[Rukhaiya2004]

Hi @bigPYJ1151 , I've addressed the buffer overflow issue found by the code review bot and pushed the fix.

Summary of changes since your approval:

1. Fixed buffer overflow in FP16Vec16::save (added std::max(0, ...) clamp)
2. Updated branch with latest main
3. CI is re-running

Status:

• Original approval: Approved by you
• Bot review: Fixed buffer overflow
• CI failures: 2 unrelated test failures (tests/utils_/test_import_utils.py)
• 81/83 checks: Passed

Could you please review the buffer overflow fix and advise on the unrelated CI failures? Can we proceed with merge?

Thank you!