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vllm.v1.attention.backends.fa_utils

logger module-attribute 

logger = init_logger(__name__)

flash_attn_supports_fp8 

flash_attn_supports_fp8() -> bool
Source code in vllm/v1/attention/backends/fa_utils.py 
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def flash_attn_supports_fp8() -> bool:
    return (
        get_flash_attn_version() == 3
        and current_platform.is_device_capability_family(90)
    )

flash_attn_supports_mla 

flash_attn_supports_mla()
Source code in vllm/v1/attention/backends/fa_utils.py 
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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

flash_attn_supports_sinks 

flash_attn_supports_sinks() -> bool
Source code in vllm/v1/attention/backends/fa_utils.py 
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def flash_attn_supports_sinks() -> bool:
    if current_platform.is_xpu():
        return True
    else:
        return get_flash_attn_version() == 3

get_flash_attn_version 

get_flash_attn_version(
    requires_alibi: bool = False,
) -> int | None
Source code in vllm/v1/attention/backends/fa_utils.py 
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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

is_flash_attn_varlen_func_available 

is_flash_attn_varlen_func_available() -> bool
Source code in vllm/v1/attention/backends/fa_utils.py 
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def is_flash_attn_varlen_func_available() -> bool:
    return current_platform.is_cuda() or current_platform.is_xpu()