Fix: prefer staging views for host tensor access

[TaoZQY]

Summary

• Prefer existing staging host views when host graph orchestration accesses staged entry tensors.
• Leave pure output tensors unregistered because they have no readable bytes before execution; attempted host reads fail closed.
• Read the shared BGEMM configuration through get_tensor_data() so host orchestration never directly dereferences a device address.
• Add regression coverage for staging selection, mapping cleanup, and unregistered-span failures.

Performance

• Qwen3-14B HBG reports host_view_close count=0 bytes=0 across four onboard rounds.
• host_view_close is 20.12–24.73 us instead of about 255.98 ms in the issue reproduction.

Testing

• Pre-commit hooks passed
• Editable build rebuilt a2a3/a5 onboard and simulation runtimes
• C++ no-hardware suite: 117/117 passed
• a2a3sim targeted HBG/TMR regression: 3 passed
• a5sim HBG suite: 11 passed
• a2a3 onboard CI regressions: paged_attention + benchmark_bgemm, 2 passed (task_20260823_234452_297495916244)
• a2a3 onboard Qwen3-14B HBG, 4 rounds: 1 passed (task_20260823_195845_151406924)

Fixes #1848

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📝 Walkthrough

Walkthrough

Changes

Host tensor access

Layer / File(s)	Summary
Fallback view selection and contract src/a2a3/runtime/host_build_graph/..., src/a5/runtime/host_build_graph/...	HostTensorAccessor::add uses a supplied fallback host view first. Device registration and mapping bookkeeping occur only when no fallback view exists. Documentation reflects this order and write-back behavior.
Host orchestration view contract src/a2a3/runtime/host_build_graph/host/runtime_maker.cpp, src/a5/runtime/host_build_graph/host/runtime_maker.cpp	Host orchestration documentation describes staging views as preferred, device mappings as fallback, and failure when neither view exists.
Fallback access validation tests/ut/cpp/a2a3/test_hbg_tensor_access.cpp	Tests count registration calls and verify that fallback views avoid registration, mappings, and unregister operations while serving staged data.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟠 High · up to 18349

The current change can expose stale or uninitialized data when host graph reads pure OUT tensors because caller memory is used without being populated from device memory. The pure-OUT path should pass a null fallback or otherwise prevent such reads in both backend implementations before merge.

Poem

I’m a rabbit with a staging view,
No mapping hops when fallback will do.
Bytes stay ready, reads run light,
No unregister at closing night.
Hop, hop—the host path is right!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes implement the issue objective by avoiding registration when staging views exist and add regression coverage for this behavior [#1848].
Out of Scope Changes check	✅ Passed	The code, documentation, and tests are directly related to the linked performance fix and contain no unrelated changes.
Docstring Coverage	✅ Passed	Docstring check was indeterminate for this PR — some files could not be analyzed in time. Not blocking.
Title check	✅ Passed	The title clearly and concisely describes the primary change: preferring staging views for host tensor access.
Description check	✅ Passed	The description directly explains the staging-view change, fallback behavior, performance impact, regression coverage, and test results.

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Actionable comments posted: 1

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Inline comments:
In `@src/a2a3/runtime/host_build_graph/host/runtime_maker.cpp`:
- Around line 993-997: Update the fallback view passed to
HostTensorAccessor::add in the tensor preparation path so pure OUT tensors
(is_pure_output) receive nullptr rather than host_ptr; retain the existing
staging or device-mapping view for other tensors and preserve the fail-closed
behavior when neither is available.

Apply the same fix in `@src/a5/runtime/host_build_graph/host/runtime_maker.cpp`
around lines 1009 - 1013: The same pure-OUT fallback issue and remediation exist
in the mirrored a5 implementation.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 5acd032b-bba0-4331-b125-6322f128a441

📥 Commits

Reviewing files that changed from the base of the PR and between 969c6a6 and 1834971.

📒 Files selected for processing (7)

• src/a2a3/runtime/host_build_graph/host/host_tensor_access.cpp
• src/a2a3/runtime/host_build_graph/host/runtime_maker.cpp
• src/a2a3/runtime/host_build_graph/runtime/host_tensor_access.h
• src/a5/runtime/host_build_graph/host/host_tensor_access.cpp
• src/a5/runtime/host_build_graph/host/runtime_maker.cpp
• src/a5/runtime/host_build_graph/runtime/host_tensor_access.h
• tests/ut/cpp/a2a3/test_hbg_tensor_access.cpp

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