device_api: run memory views through the CPU proxy from GPU kernels [CPU Proxy part 5]

[tomerdav]

What?

The GPU side of the CPU proxy, in two parts.

1. Dispatch by execution mode. A prepared memory view now records which implementation runs it, and the
   device API routes every call on that view to that implementation:
   • nixl_device_exec_mode_t (UCX_DIRECT = 1, PROXY = 2) and memViewWrapper { execution_mode, backend_memview } in device_types.cuh. Zero is deliberately invalid, so an untagged handle can never
     select a backend by accident.
   • device_memview.{h,cpp} in device_utils: nixl::deviceMemViewAllocate / GetBackend / Free, the
     owning wrapper a backend hands out from prepMemView, and nixl::deviceViewHandle, the name for it.
   • device_dispatch.cuh: nixlPut, nixlAtomicAdd, nixlGetPtr and nixlGetXferStatus dispatch on the mode.
   • level_utils.cuh: laneId, isLeader, sync and broadcastStatus for the thread/warp/block/grid levels,
     shared by the dispatch layer and the implementations.
   • The UCX engine wraps its direct views as UCX_DIRECT; one prepMemViewImpl serves the local and remote
     overloads.
2. The proxy arm. impl/proxy/device_proxy.cuh and device_proxy_ring.cuh: the GPU producer that turns a
   device API call on a PROXY view into a 64-byte ring command for the host runtime (utils: add the device proxy runtime and its worker threads [CPU Proxy part 3] #2223), and polls the
   ring's completion slot for its status.

Nothing creates a PROXY view yet; the UCX backend does that in part 6 (#2327). Existing users see no change:
every view is still a direct UCX view, and kernels are unchanged.

This PR carries production code only. Its tests, including the real-GPU ProxyDeviceApiTest suite, are in the
tests PR #2283, the last PR of the stack (on part 6, #2327).

Why?

The CPU proxy runs the same device API calls through a different implementation. Kernel code must not change
between the two, so the choice has to travel with the memory view, and the transfer status has to remember it
for polling.

How?

Dispatch

• A put or atomic add dispatches on the destination view's mode: the destination is the view that names the
  peer.
• On an accepted submission (NIXL_IN_PROG), the leader writes the mode into the transfer status's mode
  field (after the implementation's 60-byte payload), behind a barrier at every level, so a poll in the same
  kernel never reads it untagged. nixlGetXferStatus dispatches on that stored mode.
• nixlGetPtr dispatches on the view's mode, and returns null for a mode that offers no direct pointer.
• Optional compile-time mode. nixlPut, nixlAtomicAdd, nixlGpuGetXferStatus and nixlGetPtr take a mode
  template argument after the level, defaulting to DYNAMIC (the runtime selection above), e.g.
  nixlPut<nixl_gpu_level_t::WARP, nixl_device_exec_mode_t::PROXY>(...). A fixed mode compiles out the other
  implementation, the mode load, and the status tag with its barrier. It must match the backend's views (a
  mismatch is undefined, asserted in debug builds), and its status must be polled with the same mode. Fixing
  UCX_DIRECT without UCX's GPU device API is a compile error. For a warp-level put, atomic add, poll and
  get-pointer kernel, the optimized code is 2120 instructions with DYNAMIC, 1520 with UCX_DIRECT (-28%) and
  496 with PROXY (-77%).
• The proxy arm needs only CUDA, so unlike the UCX arm it is always compiled in, with no capability gate.
• The UCX plugin always links device_utils, which builds without CUDA; without a device implementation loaded,
  prepMemView returns NIXL_ERR_NOT_SUPPORTED. There is no compile-time guard.
• Device-side submission and completion errors now reach the host in the integration tests instead of being
  dropped.

Producer

• The GPU never dereferences a host view: a command carries the view tokens from utils: add the proxy memory-view manager [CPU Proxy part 2] #2226 and the element
  index/offset, and the worker resolves them.
• A command goes to the ring of its (channel, peer). The producer claims a ticket, waits while the ring is full
  (reading a device-cached consumer index, refreshed from the published consumer index only when the ring looks
  full), writes the command, and publishes it with a system-scope release store of its op index. A runtime
  shutdown word turns a blocked producer into an error instead of a hang; the shutdown drain retires the ticket
  it gave up on.
• Collective levels follow UCX: at warp and block level every thread returns the leader's submission and
  completion status. At grid level, which needs a cooperative launch, the call is a grid barrier and each thread
  polls the completion itself.
• As in UCX's device API, handle and context invariants (a null or mismatched context, a peer outside the
  runtime, a missing ring) are asserts, not returned errors. Only states a running proxy can reach return a
  status: shutdown, a full ring (which waits), and failures reported on completion.
• Status is a collapsed completion queue per ring. The host writes completion_status, then failed_idx (the
  first failed op), then completed_idx; a poll reads completed_idx before failed_idx. An op at or after the
  first failure reports that failure's status, an op up to completed_idx succeeded, and anything else is pending.
• The protocol headers are installed with the GPU API. Each proxy worker makes the CUDA device its rings were
  allocated on current before it runs, so transport calls from worker threads target the right device.

Testing

• On main at 464c8638, debug -Werror: host suite 183 pass; without CUDA, 163 pass and the build has no
  device-API define. On an L40S, ucxDeviceApi/* 9/9. CI clang-format and copyright checks clean. The only
  test change here is the existing UCX device test (single_write_test.cu) following the wrapper API.
• The tests for this PR are in test: cover the CPU proxy runtime and GPU path [CPU Proxy part 4] #2283. With PLUGIN/UCX: run the CPU proxy over UCX [CPU Proxy part 6] #2327 and test: cover the CPU proxy runtime and GPU path [CPU Proxy part 4] #2283 on top, on the same L40S with bugfix: test: make the device API tests actually run #2194's flag:
  ProxyDeviceApiTest and ucxDeviceApi 37/37 (including the fixed-mode cases), host suite 211 pass, and a
  TSAN build of the host proxy tests 24/24 with no reports.

Note: #2194

The device tests for this PR (in #2283) depend on #2194 ("test: make the device API tests actually run"). Without
it, gtest::hasCudaGpu() is compiled out in .cu test files, so every device API test reports "No CUDA-capable
GPU" and is skipped rather than run. The results above were produced with #2194's flag applied locally.

Commits

• b50b773f device_api: tag a prepared memory view with the implementation that runs it
• 183291b7 device_api: dispatch a transfer to the mode its memory view names
• 31f7f403 device_api: add the GPU-side ring producer for the CPU proxy
• 8be25bf4 device_api: take the device allocator explicitly in the memview wrappers
• 78a98cbd device_api: give every proxy thread the leader's submission status
• cf9c92cd utils: name the failed proxy op before publishing its frontier
• 1bdbfb8d utils: retire proxy tickets abandoned at shutdown
• 03f006ae device_api: stop promising mixed direct and proxied views
• 8251aa55 device_api: address the proxy device-code review
• dc0959c2 device_api: let callers fix the execution mode at compile time
• dcf2f55a PLUGIN/UCX: declare the memory-view helper in the private section
• 4e2a4652 device_api: keep the execution mode in a named status field

Based on main 464c8638, with main merged in up to f2086f56 (#2230). Part 6 (#2327) is stacked on this PR, part 7
(#2328) and the tests PR #2283 on #2327.

Changes in this revision

• New commit 4e2a4652, for review: xferStatusH is storage[60] plus a named exec_mode_t mode field
  instead of a tag in the last four bytes of storage. Still 64 bytes; backends keep the same 60-byte
  payload.

Previous revisions:

• Merged main (d52c5c36) to bring in device api: Restore user-facing Device API Doxygen comments #2230's Device API Doxygen comments. The only conflict was in
  nixl_device.cuh: device api: Restore user-facing Device API Doxygen comments #2230's comment blocks are kept with this PR's mode template parameter, and each block gains
  a @tparam mode line. No code changes.
• dcf2f55a (new commit): prepMemViewImpl was declared in the UCX engine's public section by mistake; it is
  now private, so the engine's public interface is unchanged from main.
• dc0959c2 (new commit): the optional compile-time mode template argument (see How?), requested in review.
  Existing calls keep runtime selection unchanged.
• 8251aa55, review fixes (a new commit; the commits below it are unchanged):
  • Handle and context checks in the proxy producer are asserts, as in UCX's device API.
  • proxyXferStatus, proxyDeviceContext and opIdx follow the lowerCamelCase naming rule.
  • The memview wrapper functions are in namespace nixl: nixl::deviceMemViewAllocate,
    nixl::deviceMemViewGetBackend and nixl::deviceMemViewFree.
  • HAVE_NIXL_DEVICE_API is removed: the UCX plugin always links device_utils.
• Rebased on main at 464c8638; no code changes. The tests PR test: cover the CPU proxy runtime and GPU path [CPU Proxy part 4] #2283 moved to the end of the stack, so parts 6
  and 7 no longer depend on it.
• Rebased on main at d790ad4f; no code changes.
• The tests moved out to test: cover the CPU proxy runtime and GPU path [CPU Proxy part 4] #2283, which is now stacked on this PR: proxy_write_test.cu (ProxyDeviceApiTest)
  and RetirementSelectsTheOwningDevice. This PR no longer depends on test: cover the CPU proxy runtime and GPU path [CPU Proxy part 4] #2283.
• Review fixes:
  • 78a98cbd: warp- and block-level put/atomicAdd give every thread the leader's status; grid level
    follows UCX (each thread polls itself) and the status tag is written behind a grid barrier.
  • cf9c92cd: a separate failed_idx word, so a poll never pairs one op's index with another op's status.
  • 1bdbfb8d: the shutdown drain retires tickets a producer abandoned on a full ring instead of aborting.
  • 03f006ae: exec_mode_t no longer promises mixed direct and proxied views.
• Rebased on test: cover the CPU proxy runtime and GPU path [CPU Proxy part 4] #2283's test simplification; no code changes.
• The device memview wrappers take the device allocator explicitly (the allocate, get-backend and free
  wrappers gain a deviceOps &), so a wrapper is
  always freed by the instance that allocated it. getDeviceOps() is called only by the UCX
  engine's memview prep and release and by the GPU test fixture.
• Rebased on utils: add the device proxy runtime and its worker threads [CPU Proxy part 3] #2223's drain deadline; no other changes.
• Rebased on utils: add the device proxy runtime and its worker threads [CPU Proxy part 3] #2223's review fixes: proxyWorker::start() keeps selectOwnedDevice() under the
  new status-returning start().
• Rebased on main at 40b38570 after utils: add the proxy memory-view manager [CPU Proxy part 2] #2226 merged; no code changes.
• Rebased on the review changes in utils: add the proxy memory-view manager [CPU Proxy part 2] #2226, utils: add the device proxy runtime and its worker threads [CPU Proxy part 3] #2223 and test: cover the CPU proxy runtime and GPU path [CPU Proxy part 4] #2283; no code changes.

Summary by CodeRabbit

• New Features
  • GPU operations can now use proxy execution as well as direct UCX, including transfers, atomic updates, completion checks, and pointer lookup.
  • UCX memory views can be used with the GPU device API.
• Bug Fixes
  • GPU transfer calls now report operation failures instead of always returning success.

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🤖 CI Triage Agent — Copyright Checks · commit 7dfd5c8f

TL;DR: The Copyright Checks job failed because test/gtest/device_api/meson.build was modified in this PR (2026) but its SPDX copyright header still says 2025; update the header to 2025-2026.

Full analysis

Summary: .github/workflows/copyright-check.sh reported test/gtest/device_api/meson.build (copyright year 2025 < last modified 2026) and exited 1.

Root cause: The check script derives each file's last-modified year from git log -1 --pretty="%cs" and requires the SPDX copyright end-year to be >= that year (copyright-check.sh:61-76). Commit 7dfd5c8 ("test: exercise the CPU proxy from real GPU kernels", Tomer Davidor, 2026-09-03) edited test/gtest/device_api/meson.build — adding the proxy_device_api_test_sources block — but left line 1 of the header at Copyright (c) 2025, so end_year=2025 < last_modified=2026. This is a genuine code-hygiene violation in the PR, not CI flakiness; no other files failed.

Implicated commit: [REDACTED:Hex High Entropy String] — Tomer Davidor

File: test/gtest/device_api/meson.build:1

Suggested fix: Update the copyright line to span the range:

# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.

Then re-check the rest of the PR's touched files the same way before pushing — run .github/workflows/copyright-check.sh locally, since any other file edited in this branch whose header still ends at 2025 will fail identically.

Related: PR #2326 (branch cpu-proxy-4-gpu-producer); no existing issue found for this check.

🛡️ This comment had 1 potential secret(s) redacted (Hex High Entropy String). See request_id 983531a4-4114-4caf-a6f8-a8822b15e238 in the triage console for the audit trail.

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• src/plugins/ucx/ucx_backend.h
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• src/utils/device/proxy/proxy_runtime.cpp
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📝 Walkthrough

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Walkthrough

The change adds a GPU proxy transfer path alongside UCX direct transfers. It introduces device memory-view wrappers, host-side proxy rings and workers, execution-mode dispatch, and tests for transfer submission, completion, memory-view lifecycle, and build selection.

Changes

GPU proxy transfer path

Layer / File(s)	Summary
Device memory views and UCX integration src/api/device/gpu/device_types.cuh, src/utils/device/device_memview.*, src/plugins/ucx/ucx_backend.*, meson.build, src/utils/device/meson.build, src/plugins/ucx/meson.build, examples/device/ep/meson.build	Adds execution-mode and memory-view wrapper types and operations. UCX memory views use the wrapper in UCX_DIRECT mode. Updates device API include paths and CUDA build definitions.
Proxy rings, channels, and workers src/utils/device/proxy/proxy_ring.*, src/utils/device/proxy/proxy_channel.*, src/utils/device/proxy/proxy_worker.*, src/utils/device/proxy/meson.build	Adds per-peer rings, command submission and ordered completion publication, worker progression, and ring drain/rearm operations.
Proxy runtime and memory-view lifecycle src/utils/device/proxy/proxy_runtime.*, test/gtest/mocks/proxy_mocks.h, test/gtest/unit/util/proxy_*.cpp, test/gtest/unit/util/meson.build	Adds runtime setup, validation, memory-view preparation and retirement, worker startup, draining, and shutdown. Adds tests for runtime behavior, memory-view resolution, rollback, and lifetime.
GPU proxy dispatch and API tests src/api/device/gpu/impl/*, src/api/device/gpu/meson.build, test/gtest/device_api/*, test/gtest/meson.build	Dispatches GPU operations by execution mode and adds proxy enqueueing and status polling. Adds proxy and UCX device API test coverage and updates test build selection.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant GPUKernel
  participant ProxyDeviceContext
  participant proxyWorker
  participant proxyChannel
  participant proxyTransport
  GPUKernel->>ProxyDeviceContext: Enqueue a proxy command
  proxyWorker->>proxyChannel: Submit ready ring commands
  proxyChannel->>proxyTransport: Submit prepared request
  proxyWorker->>proxyChannel: Progress transport and publish completions
  GPUKernel->>ProxyDeviceContext: Poll transfer status

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Merge Risk: 🟡 Moderate · up to d7fa3

The copyright check is reported failing, and several proxy-path correctness concerns remain open. Fix the CI failure and resolve or explicitly accept those concerns before merging.

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 10.76% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 288 functions across 41 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		The description identifies the relevant stacked and dependent pull requests, including #2223, #2283, #2327, and #2194.
Out of Scope Changes check		The build, device API, proxy runtime, and related test changes support the stated objective of routing GPU device API operations through the CPU proxy.
Title check		The title clearly and concisely identifies the main change: routing GPU device API memory views through the CPU proxy.
Description check		The description includes complete What, Why, and How sections. It explains the execution-mode dispatch, proxy producer design, compatibility impact, testing, dependencies, and revision history.

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⚠️ Outside diff range comments (1)

🟡 Minor · Update the copyright year range to fix the failing copyright check. · meson.build:1

test/gtest/device_api/meson.build:1
📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Update the copyright year range to fix the failing copyright check.

This PR modifies the file in 2026, but the SPDX header still says 2025. The Copyright Checks job fails on this line.

🔧 Proposed fix

-# SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.

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Review comment at @test/gtest/device_api/meson.build at line 1:
Update the SPDX copyright year in the file header from 2025 to the range
2025-2026 to satisfy the copyright check; leave the remaining header text
unchanged.

Source: Pipeline failures

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Inline comments:
Review comments at @src/api/device/gpu/impl/device_dispatch.cuh:
- Around line 113-127: Update the put dispatch logic to reject mismatched
execution modes: compare src_view->execution_mode with dst_view->execution_mode
and return NIXL_ERR_INVALID_PARAM when they differ. Keep the existing UCX_DIRECT
and PROXY dispatch based on dst_view->execution_mode for matching modes.

Review comments at @src/api/device/gpu/impl/proxy/device_proxy_ring.cuh:
- Around line 90-98: The producer wait loop can return after claiming a ticket,
leaving it unpublished and preventing the ring from draining. Update the loop
using ticket and cached_consumer_idx so a claimed ticket always reaches the
ring: keep waiting for space after shutdown, or publish a command that the host
retires as an error before returning.

Review comments at @src/api/device/gpu/impl/proxy/device_proxy.cuh:
- Around line 57-80: Update put and atomicAdd to share the leader’s enqueue
result with participating threads, matching the status-sharing behavior in
getXferStatus for WARP and BLOCK. Also prevent the proxy path from using GRID
unless it has a valid launch and status-sharing contract; avoid relying on
sync<level>() for unsupported GRID execution.

Review comments at @src/utils/device/host_published_mem.h:
- Line 38: Rename the enumerators in host_published_mem::backing to upper snake
case and update all references to backing::device and backing::mapped_host in
the listed implementation and test code. Also rename the enumerators in
proxyRuntime::state to upper snake case and update their uses.

Review comments at @src/utils/device/proxy/proxy_ring.cpp:
- Around line 117-120: Update the completion protocol used by retireOldest and
pollXferStatus so each error status is associated with the operation that
produced it; publish and read that association atomically rather than allowing a
later operation’s error to affect an earlier poll. Reset the error association
in rearm and update the completion-slot layout static_asserts to match.

---

Outside diff comments:
Review comments at @test/gtest/device_api/meson.build:
- Line 1: Update the SPDX copyright year in the file header from 2025 to the
range 2025-2026 to satisfy the copyright check; leave the remaining header text
unchanged.

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• src/api/device/gpu/device_types.cuh
• src/api/device/gpu/impl/device_dispatch.cuh
• src/api/device/gpu/impl/level_utils.cuh
• src/api/device/gpu/impl/proxy/device_proxy.cuh
• src/api/device/gpu/impl/proxy/device_proxy_ring.cuh
• src/api/device/gpu/meson.build
• src/meson.build
• src/plugins/ucx/meson.build
• src/plugins/ucx/ucx_backend.cpp
• src/plugins/ucx/ucx_backend.h
• src/utils/device/device_memview.cpp
• src/utils/device/device_memview.h
• src/utils/device/device_ops.h
• src/utils/device/device_ops_cuda.cpp
• src/utils/device/device_ops_cuda_gdrcopy.cpp
• src/utils/device/device_ops_cuda_gdrcopy.h
• src/utils/device/host_published_mem.cpp
• src/utils/device/host_published_mem.h
• src/utils/device/meson.build
• src/utils/device/proxy/README.md
• src/utils/device/proxy/meson.build
• src/utils/device/proxy/proxy_channel.cpp
• src/utils/device/proxy/proxy_channel.h
• src/utils/device/proxy/proxy_config.cpp
• src/utils/device/proxy/proxy_config.h
• src/utils/device/proxy/proxy_control_buffer.cpp
• src/utils/device/proxy/proxy_control_buffer.h
• src/utils/device/proxy/proxy_protocol.h
• src/utils/device/proxy/proxy_registry.cpp
• src/utils/device/proxy/proxy_registry.h
• src/utils/device/proxy/proxy_registry_entry.h
• src/utils/device/proxy/proxy_ring.cpp
• src/utils/device/proxy/proxy_ring.h
• src/utils/device/proxy/proxy_runtime.cpp
• src/utils/device/proxy/proxy_runtime.h
• src/utils/device/proxy/proxy_submission.cpp
• src/utils/device/proxy/proxy_submission.h
• src/utils/device/proxy/proxy_transport.h
• src/utils/device/proxy/proxy_worker.cpp
• src/utils/device/proxy/proxy_worker.h
• src/utils/meson.build
• test/gtest/device_api/meson.build
• test/gtest/device_api/proxy_write_test.cu
• test/gtest/device_api/single_write_test.cu
• test/gtest/meson.build
• test/gtest/mocks/proxy_mocks.h
• test/gtest/unit/device_ops/host_published_mem.cpp
• test/gtest/unit/device_ops/meson.build
• test/gtest/unit/util/meson.build
• test/gtest/unit/util/proxy_memview_registry.cpp
• test/gtest/unit/util/proxy_registry_lifetime.cpp
• test/gtest/unit/util/proxy_runtime.cpp

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Inline comments:
Review comments at @test/gtest/unit/util/proxy_memview_manager_lifetime.cpp:
- Line 20: Value-initialize `nixlProxyDeviceContextData context` and
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• src/utils/device/proxy/proxy_memview_manager.h
• src/utils/device/proxy/proxy_runtime.cpp
• src/utils/device/proxy/proxy_runtime.h
• src/utils/device/proxy/proxy_submission.cpp
• src/utils/device/proxy/proxy_submission.h
• src/utils/device/proxy/proxy_transport.h
• test/gtest/mocks/proxy_mocks.h
• test/gtest/unit/util/meson.build
• test/gtest/unit/util/proxy_memview_manager.cpp
• test/gtest/unit/util/proxy_memview_manager_lifetime.cpp
• test/gtest/unit/util/proxy_runtime.cpp

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[tomerdav]

/build

[tomerdav]

/build

[tomerdav]

/build

[ovidiusm]

/ok to test 8de562b

[rakhmets]

I think we can remove it. We added aliases for the old C-style names. We do not need aliases for new names and types.

[tomerdav]

Done in eccd56e7 (#2464).

[rakhmets]

This is a suboptimal use of a template variable. it should be smth like:

    if constexpr (mode == exec_mode_t::UCX_DIRECT) {
        return ucx::put();
    } else if constexpr (mode == exec_mode_t::PROXY) {
        return proxy::put();
    } else {
        if (runtime_mode == exec_mode_t::UCX_DIRECT) {
            return ucx::put();
        }
        assert(runtime_mode == exec_mode_t::PROXY);
        return proxy::put();
    }

[tomerdav]

Done in 96a807f9 (#2463): a fixed mode dispatches with if constexpr, so the other backend isn't instantiated, and only DYNAMIC branches on the tag; the same for atomicAdd, getXferStatus and getPtr. In the DYNAMIC branch I kept the NIXL_ERR_INVALID_PARAM result (nullptr from getPtr) for an untagged view instead of the assert, since a zero tag means an invalid view, not a backend.