[Perf][Diffusion] MiniMax-H3: opt into packed varlen attention on NPU to eliminate quadratic mask materialization (#5891)

Signed-off-by: brandneway <gyuan4892@gmail.com>

recipes/MiniMaxAI/MiniMax-H3-NPU.md

@@ -79,7 +79,8 @@ sharing the tokenizer, processor, text encoder, and VAEs from `FL2VA`.
 ### Multi-NPU: 768P combined-service configuration
 
 Use Ulysses sequence parallelism degree 8, text-encoder tensor parallelism
-degree 8, native tiled VAE patch parallelism degree 8, and layerwise offload:
+degree 8, native tiled VAE patch parallelism degree 8, and distributed
+layerwise offload:
 
 ```bash
 export ASCEND_RT_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
@@ -98,7 +99,7 @@ vllm serve "${MODEL}" \
   --usp 8 \
   --ring 1 \
   --text-encoder-tp-size 8 \
-  --enable-layerwise-offload \
+  --enable-distributed-layerwise-offload \
   --vae-parallel-mode tile \
   --vae-use-tiling \
   --vae-patch-parallel-size 8 \
@@ -112,6 +113,11 @@ H3 is CFG-distilled, so `--cfg-parallel-size` must remain 1.
 The same endpoint accepts `task=t2va`, `task=fl2va`, and `task=ref2va`; no
 partition restart is required. Layerwise offload applies to both DiTs.
 
+On Atlas 800I A3 (64 GB HBM per device) the combined service does not fit at
+768P without offloading or sharding: use distributed layerwise offload (as
+above) or HSDP — see
+[§ Memory and attention optimizations](#memory-and-attention-optimizations-a3).
+
 ### Optional optimizations
 
 Two independent optimizations may be enabled on top of the configuration
@@ -134,6 +140,63 @@ Keep `--ring 1` when using RainFusion: the `rf_v2` kernel ranks key blocks
 over the whole sequence, so ring parallelism would split away the keys it
 needs. Scale with `--usp` instead.
 
+## Memory and attention optimizations (A3)
+
+### Fitting 768P into 64 GB HBM: distributed layerwise offload or HSDP
+
+Each Atlas 800I A3 NPU has 64 GB of HBM, which is not enough for the
+combined MiniMax-H3 service at 768P. Enable **one** of the following at
+server startup:
+
+**Distributed layerwise offload** — DiT layers are offloaded with parameters
+gathered across the parallel group instead of replicated per rank:
+
+```bash
+  --enable-distributed-layerwise-offload
+```
+
+Measured peak NPU memory is about 45 GB per device for Ref2VA with a 13.88 s
+input video generating a 15 s 768P (1344x768) output video. Host (CPU) memory
+usage is high with this option.
+
+**HSDP** — hybrid sharded data parallelism for the DiT parameters. The
+multi-stream memory-reuse knob is mandatory with this flag:
+
+```bash
+export MULTI_STREAM_MEMORY_REUSE=2
+```
+
+```bash
+  --use-hsdp
+```
+
+Host memory usage is small, but large shapes may still OOM; HBM usage
+optimizations for this path are ongoing.
+
+### FLASH_ATTN backend with MindIE-SD
+
+Keep `--diffusion-attention-backend FLASH_ATTN` and install MindIE-SD (see
+[§ Environment](#environment)). On NPU this backend carries most of the
+memory-reduction and performance work: a mask-free packed varlen path that
+never materializes the quadratic `full_qk` padding mask, and K/V prefix
+slicing, both driven by the packed `cu_seqlens` metadata emitted by the H3
+transformer.
+
+### Optional: LaserAttention fused kernel
+
+For an additional attention speedup, select the Ascend LaserAttention fused
+kernel before starting the server:
+
+```bash
+export MINDIE_SD_FA_TYPE="ascend_laser_attention"
+```
+
+This requires the FLASH_ATTN backend and MindIE-SD. H3 automatically applies
+exact power-of-two input pre-scaling (`laser_input_scale=256`) so the
+kernel's fp16 workspace cannot overflow on outlier activations. Measured
+speedup numbers will be added here.
+
+
 ## HTTP API examples
 
 The request format is identical to the GPU recipe; see
@@ -154,8 +217,8 @@ configuration above:
 
 | Workload | Configuration |
 |----------|---------------|
-| T2VA, 209 frames, 1344x768 | TE TP8, layerwise offload, Ulysses 8, VPP8 tile, regional compile |
-| Ref2VA (prompt + video), 124 frames, 1344x768 | TE TP8, layerwise offload, Ulysses 8, VPP8 tile, regional compile |
+| T2VA, 209 frames, 1344x768 | TE TP8, distributed layerwise offload, Ulysses 8, VPP8 tile, regional compile |
+| Ref2VA (prompt + video), 124 frames, 1344x768 | TE TP8, distributed layerwise offload, Ulysses 8, VPP8 tile, regional compile |
 
 These measurements describe the validated shapes rather than a general
 throughput guarantee.

tests/diffusion/attention/test_flash_attn.py

@@ -9,6 +9,10 @@ This script tests two main scenarios:
 2. Case 2: Comparing FlashAttention and SDPA backends for batch_size=2 with padding
 """
 
+import sys
+from types import SimpleNamespace
+from unittest.mock import Mock
+
 import pytest
 import torch
 
@@ -371,6 +375,264 @@ def test_packed_varlen_metadata_must_be_complete(monkeypatch):
         impl.forward_cuda(query, query, query, metadata)
 
 
+# ---------------------------------------------------------------------------
+# NPU mask-free packed attention (PR #5891).
+#
+# These tests cover the backend-side NPU branches that ``forward_fa_npu`` adds:
+#   - boundary resolution in ``_resolve_packed_seq_npu`` (pure Python, no device
+#     sync, no MindIE-SD import);
+#   - env dispatch in ``forward_fa_npu`` (which op a packed forward takes);
+#   - the laser input pre-scaling math in ``_forward_prefix_kv_slice_npu``.
+#
+# They run on CPU without a real NPU by injecting a fake ``mindiesd`` module
+# into ``sys.modules`` (same pattern as ``test_paged_attention`` /
+# ``benchmarks/conftest``). No production code is exercised for real kernels.
+
+
+def _npu_impl(causal: bool = False) -> FlashAttentionImpl:
+    return FlashAttentionImpl(num_heads=2, head_size=4, softmax_scale=0.5, causal=causal)
+
+
+def _fake_mindiesd(monkeypatch, *, attention_forward=None, attention_forward_varlen=None):
+    """Install a stub ``mindiesd`` so local ``from mindiesd import ...`` works.
+
+    Restored automatically by monkeypatch. Defaults are no-op callables so the
+    top-level import in ``forward_fa_npu`` succeeds even when the test mocks the
+    instance methods that would otherwise call them.
+    """
+    fake = SimpleNamespace(
+        attention_forward=attention_forward or Mock(return_value=torch.zeros(1)),
+        attention_forward_varlen=attention_forward_varlen or Mock(return_value=torch.zeros(1)),
+    )
+    monkeypatch.setitem(sys.modules, "mindiesd", fake)
+    return fake
+
+
+# --- Test group A: boundary resolution (_resolve_packed_seq_npu) -------------
+
+
+def test_resolve_packed_seq_accepts_real_plus_pad_contract():
+    impl = _npu_impl()
+    q = torch.randn(1, 8, 2, 4)
+    cu = torch.tensor([0, 5, 8], dtype=torch.int32)
+    extra = {"cu_seqlens_q": cu, "cu_seqlens_k": cu, "max_seqlen_q": 5, "max_seqlen_k": 5}
+
+    assert impl._resolve_packed_seq_npu(q, q, extra) == ([5, 8], [5, 8])
+
+
+def test_resolve_packed_seq_single_document_no_padding():
+    impl = _npu_impl()
+    q = torch.randn(1, 8, 2, 4)
+    cu = torch.tensor([0, 8], dtype=torch.int32)
+    extra = {"cu_seqlens_q": cu, "cu_seqlens_k": cu, "max_seqlen_q": 8, "max_seqlen_k": 8}
+
+    assert impl._resolve_packed_seq_npu(q, q, extra) == ([8], [8])
+
+
+@pytest.mark.parametrize(
+    "case",
+    [
+        "causal",
+        "missing_key",
+        "batch_ne_1",
+        "cu_too_long",
+        "cu_qk_mismatch",
+        "max_seqlen_not_int",
+        "used_gt_total",
+        "used_zero",
+        "pad_longer_than_real",
+    ],
+)
+def test_resolve_packed_seq_rejects_malformed_metadata(case):
+    """Any metadata outside the exact [real, pad] contract returns None so the
+    caller falls back to the masked path."""
+    cu3 = torch.tensor([0, 5, 8], dtype=torch.int32)
+    cu2 = torch.tensor([0, 8], dtype=torch.int32)
+    cu4 = torch.tensor([0, 3, 5, 8], dtype=torch.int32)
+    cu_short = torch.tensor([0, 3, 8], dtype=torch.int32)
+    base = {"cu_seqlens_q": cu3, "cu_seqlens_k": cu3, "max_seqlen_q": 5, "max_seqlen_k": 5}
+    q = torch.randn(1, 8, 2, 4)
+
+    impl = _npu_impl(causal=(case == "causal"))
+    if case == "missing_key":
+        extra = {"cu_seqlens_q": cu3, "max_seqlen_q": 5}  # 3 of 4 keys absent
+    elif case == "batch_ne_1":
+        q = torch.randn(2, 8, 2, 4)
+        extra = base
+    elif case == "cu_too_long":
+        extra = {**base, "cu_seqlens_q": cu4, "cu_seqlens_k": cu4}
+    elif case == "cu_qk_mismatch":
+        extra = {**base, "cu_seqlens_q": cu3, "cu_seqlens_k": cu2}
+    elif case == "max_seqlen_not_int":
+        extra = {**base, "max_seqlen_q": 5.0}
+    elif case == "used_gt_total":
+        extra = {**base, "max_seqlen_q": 9}  # 9 > total length 8
+    elif case == "used_zero":
+        extra = {**base, "max_seqlen_q": 0, "max_seqlen_k": 0}
+    elif case == "pad_longer_than_real":
+        # real=3, pad=5: the real document must be the longer one.
+        extra = {
+            "cu_seqlens_q": cu_short,
+            "cu_seqlens_k": cu_short,
+            "max_seqlen_q": 3,
+            "max_seqlen_k": 3,
+        }
+    else:
+        extra = base
+
+    assert impl._resolve_packed_seq_npu(q, q, extra) is None
+
+
+# --- Test group B: env dispatch in forward_fa_npu (current behavior) --------
+
+
+def _npu_impl_with_mocked_paths() -> tuple[FlashAttentionImpl, torch.Tensor]:
+    """FlashAttentionImpl whose two packed paths are Mocks returning a sentinel.
+
+    Lets a test assert which branch ``forward_fa_npu`` dispatched to without
+    running any real kernel.
+    """
+    impl = _npu_impl()
+    sentinel = torch.tensor([1.0])
+    impl._forward_varlen_packed_npu = Mock(return_value=sentinel)
+    impl._forward_prefix_kv_slice_npu = Mock(return_value=sentinel)
+    return impl, sentinel
+
+
+def test_npu_env_unset_uses_varlen(monkeypatch):
+    monkeypatch.delenv("MINDIE_SD_FA_TYPE", raising=False)
+    _fake_mindiesd(monkeypatch)
+    impl, _ = _npu_impl_with_mocked_paths()
+    metadata = AttentionMetadata(extra={"npu_attn_varlen": True})
+
+    impl.forward_fa_npu(torch.randn(1, 8, 2, 4), *[torch.randn(1, 8, 2, 4)] * 2, metadata)
+
+    impl._forward_varlen_packed_npu.assert_called_once()
+    impl._forward_prefix_kv_slice_npu.assert_not_called()
+
+
+def test_npu_env_laser_uses_slice(monkeypatch):
+    monkeypatch.setenv("MINDIE_SD_FA_TYPE", "ascend_laser_attention")
+    _fake_mindiesd(monkeypatch)
+    impl, _ = _npu_impl_with_mocked_paths()
+    metadata = AttentionMetadata(extra={"npu_attn_varlen": True})
+
+    impl.forward_fa_npu(torch.randn(1, 8, 2, 4), *[torch.randn(1, 8, 2, 4)] * 2, metadata)
+
+    impl._forward_prefix_kv_slice_npu.assert_called_once()
+    impl._forward_varlen_packed_npu.assert_not_called()
+
+
+@pytest.mark.parametrize("fa_type", ["prompt_flash_attn", "fused_attn_score", "ascend_laser_attenton", "garbage"])
+def test_npu_env_non_laser_value_falls_back_to_varlen(monkeypatch, fa_type):
+    """Locks in the CURRENT dispatch behavior (PR #5891).
+
+    Any value other than ``ascend_laser_attention`` — including the valid
+    ``prompt_flash_attn``/``fused_attn_score`` and typos — is silently routed to
+    the TND varlen op, which cannot honor ``MINDIE_SD_FA_TYPE``. This is review
+    comment #1 on the PR and is intentionally captured here as a regression
+    net: update this test when the dispatch starts honoring/rejecting these.
+    """
+    monkeypatch.setenv("MINDIE_SD_FA_TYPE", fa_type)
+    _fake_mindiesd(monkeypatch)
+    impl, _ = _npu_impl_with_mocked_paths()
+    metadata = AttentionMetadata(extra={"npu_attn_varlen": True})
+
+    impl.forward_fa_npu(torch.randn(1, 8, 2, 4), *[torch.randn(1, 8, 2, 4)] * 2, metadata)
+
+    impl._forward_varlen_packed_npu.assert_called_once()
+    impl._forward_prefix_kv_slice_npu.assert_not_called()
+
+
+def test_npu_varlen_opt_in_unset_takes_mask_path(monkeypatch):
+    """Models that do not set ``npu_attn_varlen`` (e.g. Wan/Cosmos) keep using
+    the existing masked attention_forward path; the new branches are skipped."""
+    fake_forward = Mock(return_value=torch.zeros(1, 8, 2, 4))
+    _fake_mindiesd(monkeypatch, attention_forward=fake_forward)
+    impl, _ = _npu_impl_with_mocked_paths()
+    metadata = AttentionMetadata(
+        attn_mask=torch.tensor([[True, True, True, True, True, False, False, False]]),
+        extra={},  # no npu_attn_varlen opt-in
+    )
+    q = torch.randn(1, 8, 2, 4)
+
+    out = impl.forward_fa_npu(q, q, q, metadata)
+
+    impl._forward_varlen_packed_npu.assert_not_called()
+    impl._forward_prefix_kv_slice_npu.assert_not_called()
+    fake_forward.assert_called_once()
+    assert out is fake_forward.return_value
+
+
+# --- Test group C: laser input pre-scaling in _forward_prefix_kv_slice_npu --
+
+
+def test_prefix_kv_slice_applies_laser_input_scaling(monkeypatch):
+    monkeypatch.setenv("MINDIE_SD_FA_TYPE", "ascend_laser_attention")
+    captured: dict = {}
+
+    def fake_attention_forward(query, key, value, **kwargs):
+        captured["query"] = query
+        captured["key"] = key
+        captured["value"] = value
+        captured.update(kwargs)
+        return torch.ones_like(query)  # deterministic stand-in kernel output
+
+    _fake_mindiesd(monkeypatch, attention_forward=fake_attention_forward)
+    impl = _npu_impl()  # softmax_scale == 0.5
+    q = torch.randn(1, 8, 2, 4)
+    cu = torch.tensor([0, 5, 8], dtype=torch.int32)
+    extra = {
+        "cu_seqlens_q": cu,
+        "cu_seqlens_k": cu,
+        "max_seqlen_q": 5,
+        "max_seqlen_k": 5,
+        "laser_input_scale": 256.0,
+    }
+
+    out = impl._forward_prefix_kv_slice_npu(q, q, q, extra)
+
+    # K/V sliced to the valid prefix (used_k == 5); q keeps full length.
+    assert captured["key"].shape == (1, 5, 2, 4)
+    assert captured["value"].shape == (1, 5, 2, 4)
+    assert captured["query"].shape == (1, 8, 2, 4)
+    assert captured["attn_mask"] is None
+    # Pre-divide q/k/v by the factor and compensate the kernel scale by its
+    # square (exact for power-of-two); output scaled back by the factor.
+    torch.testing.assert_close(captured["query"], q / 256.0)
+    torch.testing.assert_close(captured["key"], q[:, :5] / 256.0)
+    assert captured["scale"] == pytest.approx(0.5 * 256.0**2)
+    torch.testing.assert_close(out, torch.ones(1, 8, 2, 4) * 256.0)
+
+
+def test_prefix_kv_slice_no_scaling_without_factor(monkeypatch):
+    monkeypatch.setenv("MINDIE_SD_FA_TYPE", "ascend_laser_attention")
+    captured: dict = {}
+
+    def fake_attention_forward(query, key, value, **kwargs):
+        captured["query"] = query
+        captured.update(kwargs)
+        return torch.ones_like(query)
+
+    _fake_mindiesd(monkeypatch, attention_forward=fake_attention_forward)
+    impl = _npu_impl()
+    q = torch.randn(1, 8, 2, 4)
+    cu = torch.tensor([0, 5, 8], dtype=torch.int32)
+    extra = {
+        "cu_seqlens_q": cu,
+        "cu_seqlens_k": cu,
+        "max_seqlen_q": 5,
+        "max_seqlen_k": 5,
+        # no laser_input_scale → no pre-scaling
+    }
+
+    out = impl._forward_prefix_kv_slice_npu(q, q, q, extra)
+
+    torch.testing.assert_close(captured["query"], q)
+    assert captured["scale"] == pytest.approx(0.5)
+    torch.testing.assert_close(out, torch.ones(1, 8, 2, 4))
+
+
 if __name__ == "__main__":
     print("Running FlashAttention Padding Tests...")
     print("=" * 60)

tests/diffusion/models/minimax_h3/test_minimax_h3_contract.py

@@ -492,6 +492,10 @@ def test_cudnn_packed_attention_uses_python_length_without_padding_mask():
     class FakeBackend:
         supports_prefix_kv_slicing = True
 
+        @classmethod
+        def supports_packed_mask_free(cls) -> bool:
+            return False
+
     class FakeAttention(torch.nn.Module):
         attn_backend = FakeBackend
 
@@ -526,6 +530,48 @@ def test_cudnn_packed_attention_uses_python_length_without_padding_mask():
     assert attention.attention.metadata.extra["valid_kv_length"] == 5
 
 
+def test_packed_attention_skips_mask_for_packed_mask_free_backend():
+    from vllm_omni.diffusion.models.minimax_h3.minimax_h3_transformer import (
+        MiniMaxH3Attention,
+    )
+
+    class FakeBackend:
+        supports_prefix_kv_slicing = False
+
+        @classmethod
+        def supports_packed_mask_free(cls) -> bool:
+            return True
+
+    class FakeAttention(torch.nn.Module):
+        attn_backend = FakeBackend
+
+        def __init__(self):
+            super().__init__()
+            self.metadata = None
+
+        def forward(self, query, key, value, metadata):
+            self.metadata = metadata
+            return query
+
+    attention = object.__new__(MiniMaxH3Attention)
+    torch.nn.Module.__init__(attention)
+    attention.attention = FakeAttention()
+    q = torch.randn(8, 2, 4)
+
+    attention._run_packed_attention(
+        q,
+        q,
+        q,
+        cu_seqlens=torch.tensor([0, 5, 8], dtype=torch.int32),
+        max_seqlen=5,
+        packed_total=8,
+    )
+
+    assert attention.attention.metadata.attn_mask is None
+    assert attention.attention.metadata.extra["valid_kv_length"] == 5
+    assert attention.attention.metadata.extra["npu_attn_varlen"] is True
+
+
 def test_packed_attention_keeps_padding_mask_for_other_backends():
     from vllm_omni.diffusion.models.minimax_h3.minimax_h3_transformer import (
         MiniMaxH3Attention,
@@ -534,6 +580,10 @@ def test_packed_attention_keeps_padding_mask_for_other_backends():
     class FakeBackend:
         supports_prefix_kv_slicing = False
 
+        @classmethod
+        def supports_packed_mask_free(cls) -> bool:
+            return False
+
     class FakeAttention(torch.nn.Module):
         attn_backend = FakeBackend
 

vllm_omni/diffusion/attention/backends/abstract.py

@@ -20,6 +20,17 @@ class AttentionBackend(ABC):
     # avoid a slower masked-attention plan when tail padding is not semantic.
     supports_prefix_kv_slicing: bool = False
 
+    @classmethod
+    def supports_packed_mask_free(cls) -> bool:
+        """Whether packed attention never reads attn_mask on this platform.
+
+        When True, models that pack a [real, pad] two-document layout and
+        carry cu_seqlens/max_seqlen in ``AttentionMetadata.extra`` may skip
+        constructing the padding mask entirely. Backends whose mask-free
+        behavior is platform-dependent must check current_omni_platform.
+        """
+        return False
+
     # ``OmniPlatformEnum`` values this backend runs on; None means unrestricted.
     # Platform resolution rejects an explicit selection outside this set, so a
     # hardware-specific backend fails before the model is built.
@@ -121,6 +132,17 @@ class AttentionMetadata:
     #     the packed cu_seqlens tensors.
     #   "valid_kv_length": int — contiguous valid K/V prefix length for a
     #     backend that advertises supports_prefix_kv_slicing.
+    #   "npu_attn_varlen": bool — model opt-in for the NPU packed varlen path
+    #     (TND npu_fusion_attention driven by cu_seqlens, mask never read).
+    #     Requires the [real, pad] two-document packing contract; see
+    #     FlashAttentionImpl._forward_varlen_packed_npu.
+    #   "laser_input_scale": float — model opt-in input pre-scale for the NPU
+    #     ascend_laser_attention path. The kernel stores unscaled QK^T in an
+    #     fp16 workspace, so outlier activations overflow 65504 into NaN rows;
+    #     with this set (>1), q/k/v are divided by the factor before the op,
+    #     the kernel scale_value is multiplied by its square, and the output
+    #     is scaled back (exact for power-of-two factors). Absent means no
+    #     pre-scaling. See FlashAttentionImpl._forward_prefix_kv_slice_npu.
 
     # Piecewise attention metadata (mixed causal/full masks).
     # full_attn_spans: per-sample [start, end) spans in global coordinates using full attention.

vllm_omni/diffusion/attention/backends/flash_attn.py

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import os
 from functools import partial
 
 import torch
@@ -16,6 +17,7 @@ from vllm_omni.diffusion.attention.backends.utils.piecewise_attn import (
     piecewise_attn,
 )
 from vllm_omni.diffusion.config import get_current_diffusion_config_or_none
+from vllm_omni.platforms import current_omni_platform
 
 logger = init_logger(__name__)
 
@@ -24,6 +26,15 @@ class FlashAttentionBackend(AttentionBackend):
     accept_output_buffer: bool = True
     supports_piecewise_spans: bool = True
 
+    @classmethod
+    def supports_packed_mask_free(cls) -> bool:
+        # CUDA: forward_cuda dispatches the packed cu_seqlens varlen path
+        # before ever reading attn_mask. NPU: forward_fa_npu honors the
+        # npu_attn_varlen opt-in and rebuilds the padding mask itself if the
+        # packed contract fails. XPU reads attn_mask, so models must keep
+        # constructing it there.
+        return current_omni_platform.is_cuda() or current_omni_platform.is_npu()
+
     @classmethod
     def supports_attention_mask(cls) -> bool:
         return True
@@ -415,7 +426,36 @@ class FlashAttentionImpl(AttentionImpl):
                 "For installation details, see https://gitcode.com/Ascend/MindIE-SD"
                 "Otherwise, use SDPA backend by setting DIFFUSION_ATTENTION_BACKEND=TORCH_SDPA"
             )
+        # Opt-in mask-free paths (mirror the CUDA cu_seqlens behavior): the
+        # model marks extra["npu_attn_varlen"] and carries packed metadata, so
+        # the padding document is excluded without reading or materializing
+        # the quadratic full_qk mask.
+        #   - default: packed varlen via mindiesd attention_forward_varlen;
+        #   - when MINDIE_SD_FA_TYPE=ascend_laser_attention: prefix K/V
+        #     slicing via mindiesd attention_forward (so the env-selected op
+        #     type is honored; the TND varlen op cannot be).
+        extra = attn_metadata.extra if attn_metadata else {}
+        if extra.get("npu_attn_varlen", False):
+            if os.environ.get("MINDIE_SD_FA_TYPE") == "ascend_laser_attention":
+                out = self._forward_prefix_kv_slice_npu(query, key, value, extra)
+            else:
+                out = self._forward_varlen_packed_npu(query, key, value, extra)
+            if out is not None:
+                return out
         attention_mask = attn_metadata.attn_mask if attn_metadata else None
+        if attention_mask is None and extra.get("npu_attn_varlen", False):
+            # Models skip mask construction when this opt-in is set
+            # (FlashAttentionBackend.supports_packed_mask_free). The packed
+            # paths above declined (contract mismatch), so rebuild the padding
+            # mask here to keep the masked fallback correct.
+            used = extra.get("valid_kv_length")
+            if not isinstance(used, int) or not 0 < used <= query.shape[1]:
+                raise ValueError(
+                    "npu_attn_varlen packed metadata is unusable and no attn_mask "
+                    f"was constructed (valid_kv_length={used!r}, seq_len={query.shape[1]}); "
+                    "refusing to run unmasked attention over padding rows."
+                )
+            attention_mask = torch.arange(query.shape[1], device=query.device)[None] < used
 
         # NPU aclnnFlashAttentionScore requires mask shape to be one of:
         # [B, N, Sq, Skv], [B, 1, Sq, Skv], [1, 1, Sq, Skv], or [Sq, Skv]
@@ -433,3 +473,161 @@ class FlashAttentionImpl(AttentionImpl):
             op_type="fused_attn_score",
             layout=layout,
         )
+
+    def _resolve_packed_seq_npu(
+        self,
+        query: torch.Tensor,
+        key: torch.Tensor,
+        extra: dict,
+    ) -> tuple[list[int], list[int]] | None:
+        """Resolve packed document boundaries (cumulative end offsets per doc).
+
+        Returns (seq_q, seq_k), e.g. ([used_q, total_q], [used_k, total_k]),
+        or None unless the metadata matches the exact contract MiniMax-H3
+        produces:
+          - q/k/v are [1, T, N, D] (BSND, single packed batch);
+          - cu_seqlens describe a [real, pad] two-document packing, with the
+            padding document as a strict suffix;
+          - max_seqlen_q/k are Python ints equal to the real document length
+            (so boundaries are derived without any device sync).
+        """
+        if self.causal:
+            return None
+        packed_keys = ("cu_seqlens_q", "cu_seqlens_k", "max_seqlen_q", "max_seqlen_k")
+        if any(k not in extra for k in packed_keys):
+            return None
+        cu_q, cu_k = extra["cu_seqlens_q"], extra["cu_seqlens_k"]
+        used_q, used_k = extra["max_seqlen_q"], extra["max_seqlen_k"]
+        if query.shape[0] != 1 or key.shape[0] != 1:
+            return None
+        if not isinstance(used_q, int) or not isinstance(used_k, int):
+            return None
+        total_q, total_k = query.shape[1], key.shape[1]
+        # .shape is host-side metadata: counting documents never syncs.
+        if cu_q.shape[0] != cu_k.shape[0] or cu_q.shape[0] > 3:
+            return None
+        if not (0 < used_q <= total_q) or not (0 < used_k <= total_k):
+            return None
+        if used_q == total_q and used_k == total_k:
+            # No padding document: one full-length document.
+            return [total_q], [total_k]
+        if cu_q.shape[0] == 3 and used_q >= total_q - used_q and used_k >= total_k - used_k:
+            # [real, pad] packing: the real document must be the longer one
+            # (consistent with the max_seqlen naming).
+            return [used_q, total_q], [used_k, total_k]
+        return None
+
+    def _forward_varlen_packed_npu(
+        self,
+        query: torch.Tensor,
+        key: torch.Tensor,
+        value: torch.Tensor,
+        extra: dict,
+    ) -> torch.Tensor | None:
+        """Packed varlen attention on NPU via mindiesd attention_forward_varlen.
+
+        Returns None (caller falls back to the mask path) when the packed
+        contract does not hold; see _resolve_packed_seq_npu.
+        """
+        resolved = self._resolve_packed_seq_npu(query, key, extra)
+        if resolved is None:
+            return None
+        seq_q, seq_k = resolved
+
+        try:
+            from mindiesd import attention_forward_varlen
+        except ImportError:
+            raise ImportError(
+                "FlashAttentionBackend NPU implementation requires MindIE-SD. "
+                "Please install MindIE-SD to enable NPU attention support. "
+                "For installation details, see https://gitcode.com/Ascend/MindIE-SD"
+                "Otherwise, use SDPA backend by setting DIFFUSION_ATTENTION_BACKEND=TORCH_SDPA"
+            )
+        q = query.squeeze(0)  # [T, N, D] == TND
+        k = key.squeeze(0)
+        v = value.squeeze(0)
+        # attention_forward_varlen wants cu_seqlens as host lists of cumulative
+        # offsets and forwards cu[1:] as actual_seq_qlen/actual_seq_kvlen.
+        out = attention_forward_varlen(
+            q,
+            k,
+            v,
+            [0, *seq_q],
+            [0, *seq_k],
+            softmax_scale=self.softmax_scale,
+        )
+        return out.unsqueeze(0)
+
+    def _forward_prefix_kv_slice_npu(
+        self,
+        query: torch.Tensor,
+        key: torch.Tensor,
+        value: torch.Tensor,
+        extra: dict,
+    ) -> torch.Tensor | None:
+        """Mask-free attention by slicing K/V to the valid prefix (zero-copy
+        views), then mindiesd attention_forward without a mask.
+
+        Same numerical contract as the varlen path: the padding document is a
+        strict suffix, so dropping it from K/V is identical to masking it out.
+        Query keeps full length; outputs on padding rows are never consumed
+        downstream. Returns None when the packed contract does not hold.
+
+        Used when MINDIE_SD_FA_TYPE=ascend_laser_attention: the TND varlen op
+        cannot honor the env-selected op type, while attention_forward can.
+
+        The laser kernel stores unscaled S=QK^T in an fp16 GM workspace, so
+        bf16 activations with large outliers overflow 65504 into ±inf and the
+        affected rows turn NaN. Models may opt into exact power-of-two input
+        pre-scaling via extra["laser_input_scale"] (see abstract.py): q/k/v
+        are divided by the factor, the kernel scale_value is multiplied by its
+        square, and the output is scaled back. Absent or invalid factor means
+        no pre-scaling.
+        """
+        resolved = self._resolve_packed_seq_npu(query, key, extra)
+        if resolved is None:
+            return None
+        _, seq_k = resolved
+        used_k = seq_k[0]  # real document length (first cumulative end)
+
+        try:
+            from mindiesd import attention_forward
+        except ImportError:
+            raise ImportError(
+                "FlashAttentionBackend NPU implementation requires MindIE-SD. "
+                "Please install MindIE-SD to enable NPU attention support. "
+                "For installation details, see https://gitcode.com/Ascend/MindIE-SD"
+                "Otherwise, use SDPA backend by setting DIFFUSION_ATTENTION_BACKEND=TORCH_SDPA"
+            )
+        # mindiesd always takes BSND input regardless of `layout`; the arg
+        # selects the op-internal layout and laser only supports BNSD, so do
+        # NOT forward the model's qkv_layout ("BSND" for MiniMax-H3) here.
+        layout = "BNSD"
+        key = key[:, :used_k]
+        value = value[:, :used_k]
+
+        input_scale = extra.get("laser_input_scale")
+        preserve_input_range = isinstance(input_scale, (int, float)) and input_scale > 1
+        if preserve_input_range:
+            query = query / input_scale
+            key = key / input_scale
+            value = value / input_scale
+        scale = self.softmax_scale
+        if preserve_input_range:
+            scale *= input_scale**2
+
+        def _postprocess(out: torch.Tensor) -> torch.Tensor:
+            return out * input_scale if preserve_input_range else out
+
+        return _postprocess(
+            attention_forward(
+                query,
+                key,
+                value,
+                attn_mask=None,
+                opt_mode="manual",
+                op_type="fused_attn_score",  # MINDIE_SD_FA_TYPE env overrides this
+                layout=layout,
+                scale=scale,
+            )
+        )

vllm_omni/diffusion/models/minimax_h3/minimax_h3_transformer.py

@@ -107,6 +107,15 @@ MINIMAX_H3_FP32_BUFFER_NAMES = frozenset({"rope.inv_freq"})
 # lookup and masked out afterwards).
 MINIMAX_H3_ADALN_MODALITY_NUM = 3
 
+# Opt-in fp16-range protection for the NPU ascend_laser_attention kernel
+# (consumed only via the "laser_input_scale" extra key; other backends and
+# platforms ignore it). The kernel stores unscaled QK^T in an fp16 GM
+# workspace, and H3's outlier activations (per-element amax in the hundreds)
+# push dot products past fp16 max 65504, turning whole 128-row blocks NaN.
+# 256 is a power of two, so pre-dividing q/k/v and the compensating
+# kernel-scale/output multiplies are exact in floating point.
+MINIMAX_H3_LASER_INPUT_SCALE = 256.0
+
 
 def _required_kwarg(kwargs: dict[str, Any], key: str) -> Any:
     if key not in kwargs or kwargs[key] is None:
@@ -402,11 +411,16 @@ class MiniMaxH3Attention(nn.Module):
         used = min(max_seqlen, packed_total)
         attn_mask = None
         # Ring attention can dispatch to a different implementation from the
-        # configured backend, so this no-mask fast path is local-only.
-        prefix_slice = (
-            not getattr(self.attention, "use_ring", False) and self.attention.attn_backend.supports_prefix_kv_slicing
+        # configured backend, so the no-mask fast paths are local-only.
+        # supports_prefix_kv_slicing: backend slices K/V itself (cuDNN).
+        # supports_packed_mask_free: backend consumes the packed metadata
+        # without ever reading attn_mask (CUDA packed varlen, NPU
+        # npu_attn_varlen opt-in with its own fallback rebuild).
+        no_mask = not getattr(self.attention, "use_ring", False) and (
+            self.attention.attn_backend.supports_prefix_kv_slicing
+            or self.attention.attn_backend.supports_packed_mask_free()
         )
-        if used < packed_total and not prefix_slice:
+        if used < packed_total and not no_mask:
             attn_mask = torch.arange(packed_total, device=q.device)[None] < used
         metadata = AttentionMetadata(
             attn_mask=attn_mask,
@@ -416,6 +430,15 @@ class MiniMaxH3Attention(nn.Module):
                 "max_seqlen_q": max_seqlen,
                 "max_seqlen_k": max_seqlen,
                 "valid_kv_length": used,
+                # Opt the NPU flash backend into the packed varlen path so the
+                # quadratic full_qk mask is never materialized. Ring attention
+                # is excluded: it keeps the aligned padding rows for its
+                # fixed-size P2P buffers and still needs the mask.
+                "npu_attn_varlen": not getattr(self.attention, "use_ring", False),
+                # fp16-range protection for the ascend_laser_attention kernel
+                # (see MINIMAX_H3_LASER_INPUT_SCALE). Ignored by every other
+                # backend/path.
+                "laser_input_scale": MINIMAX_H3_LASER_INPUT_SCALE,
             },
             video_layout=video_layout,
         )

vllm_omni/platforms/npu/platform.py

@@ -139,6 +139,18 @@ class NPUOmniPlatform(OmniPlatform, NPUPlatform):
                 )
                 backend_upper = "FLASH_ATTN"
 
+            if backend_upper == "FLASH_ATTN" and find_spec("mindiesd"):
+                # The NPU FLASH_ATTN backend imports mindiesd lazily at first
+                # forward, but CANN snapshots the custom-op registry at the
+                # first custom-op regInfo lookup in the process (e.g. a
+                # vllm-ascend custom op during model load/warmup). Import
+                # mindiesd here so its env.py prepends the mindiesd vendor
+                # dirs (aie_ascendc etc.) to ASCEND_CUSTOM_OPP_PATH before
+                # that snapshot; otherwise aclnnLaserAttention /
+                # FusedAttentionScore fail with EZ1001 "does not support
+                # opType" for the rest of the process.
+                import mindiesd  # noqa: F401
+
             backend = DiffusionAttentionBackendEnum[backend_upper]
             logger.debug("Using diffusion attention backend '%s'", backend_upper)
             return backend.get_path()