vllm.v1.attention.backends.fa_utils ¶

def flash_attn_supports_fp8() -> bool:
    return (
        get_flash_attn_version() == 3
        and current_platform.is_device_capability_family(90)
    )

def flash_attn_supports_mla():
    from vllm.platforms import current_platform

    if current_platform.is_cuda():
        try:
            from vllm.vllm_flash_attn.flash_attn_interface import (
                is_fa_version_supported,
            )

            return is_fa_version_supported(
                3
            ) and current_platform.is_device_capability_family(90)
        except (ImportError, AssertionError):
            pass
    return False

def flash_attn_supports_sinks() -> bool:
    if current_platform.is_xpu():
        return True
    else:
        return get_flash_attn_version() == 3

def get_flash_attn_version(requires_alibi: bool = False) -> int | None:
    # import here to avoid circular dependencies
    from vllm.platforms import current_platform

    if current_platform.is_xpu():
        return 2
    if current_platform.is_rocm():
        # ROCm doesn't use vllm_flash_attn; return None to skip fa_version arg
        return None
    try:
        from vllm.vllm_flash_attn.flash_attn_interface import (
            fa_version_unsupported_reason,
            is_fa_version_supported,
        )

        device_capability = current_platform.get_device_capability()

        assert device_capability is not None

        # 1. default version depending on platform
        fa_version = (
            3 if (device_capability.major == 9 and is_fa_version_supported(3)) else 2
        )

        # 2. override if passed by environment or config
        from vllm.config import get_current_vllm_config_or_none

        vllm_config = get_current_vllm_config_or_none()
        if (
            vllm_config is not None
            and vllm_config.attention_config.flash_attn_version is not None
        ):
            fa_version = vllm_config.attention_config.flash_attn_version

        # 3. fallback for unsupported combinations
        if device_capability.major == 10 and fa_version == 3:
            logger.warning_once(
                "Cannot use FA version 3 on Blackwell platform, "
                "defaulting to FA version 2."
            )
            fa_version = 2

        if requires_alibi and fa_version == 3:
            logger.warning_once(
                "Cannot use FA version 3 with ALiBi, defaulting to FA version 2."
            )
            fa_version = 2

        if not is_fa_version_supported(fa_version):
            logger.error(
                "Cannot use FA version %d is not supported due to %s",
                fa_version,
                fa_version_unsupported_reason(fa_version),
            )

        assert is_fa_version_supported(fa_version)
        return fa_version
    except (ImportError, AssertionError):
        return None

def is_flash_attn_varlen_func_available() -> bool:
    """Check if flash_attn_varlen_func is available.

    This function determines whether the flash_attn_varlen_func imported at module
    level is a working implementation or a stub.

    Platform-specific sources:
    - CUDA: vllm.vllm_flash_attn.flash_attn_varlen_func
    - XPU: ipex_ops.flash_attn_varlen_func
    - ROCm: upstream flash_attn.flash_attn_varlen_func (if available)

    Note: This is separate from the AITER flash attention backend (rocm_aiter_fa.py)
    which uses rocm_aiter_ops.flash_attn_varlen_func. The condition to use AITER is
    handled separately via _aiter_ops.is_aiter_found_and_supported().

    Returns:
        bool: True if a working flash_attn_varlen_func implementation is available.
    """
    if current_platform.is_cuda() or current_platform.is_xpu():
        # CUDA and XPU always have flash_attn_varlen_func available
        return True

    if current_platform.is_rocm():
        # Use the flag set during module import to check if
        # upstream flash-attn was successfully imported
        return _ROCM_FLASH_ATTN_AVAILABLE

    return False