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List:       linux-s390
Subject:    Re: [PATCH v8 1/7] iommu: enhance IOMMU default DMA mode build options
From:       John Garry <john.garry () huawei ! com>
Date:       2019-05-31 10:42:45
Message-ID: 55d0e30c-5bca-41fc-5bf0-4366dc387afd () huawei ! com
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>>> -config IOMMU_DEFAULT_PASSTHROUGH
>>> -    bool "IOMMU passthrough by default"
>>> +choice
>>> +    prompt "IOMMU default DMA mode"
>>>      depends on IOMMU_API
>>> -        help
>>> -      Enable passthrough by default, removing the need to pass in
>>> -      iommu.passthrough=on or iommu=pt through command line. If this
>>> -      is enabled, you can still disable with iommu.passthrough=off
>>> -      or iommu=nopt depending on the architecture.
>>> +    default IOMMU_DEFAULT_STRICT
>>> +    help
>>> +      This option allows IOMMU DMA mode to be chose at build time, to
>>
>> As before:
>> /s/chose/chosen/, /s/allows IOMMU/allows an IOMMU/
> I'm sorry that the previous version was not modified.
>
>>
>>> +      override the default DMA mode of each ARCHs, removing the need to
>>
>> Again, as before:
>> ARCHs should be singular
> OK
>
>>
>>> +      pass in kernel parameters through command line. You can still use
>>> +      ARCHs specific boot options to override this option again.

*

>>> +
>>> +config IOMMU_DEFAULT_PASSTHROUGH
>>> +    bool "passthrough"
>>> +    help
>>> +      In this mode, the DMA access through IOMMU without any addresses
>>> +      translation. That means, the wrong or illegal DMA access can not
>>> +      be caught, no error information will be reported.
>>>
>>>        If unsure, say N here.
>>>
>>> +config IOMMU_DEFAULT_LAZY
>>> +    bool "lazy"
>>> +    help
>>> +      Support lazy mode, where for every IOMMU DMA unmap operation, the
>>> +      flush operation of IOTLB and the free operation of IOVA are deferred.
>>> +      They are only guaranteed to be done before the related IOVA will be
>>> +      reused.
>>
>> why no advisory on how to set if unsure?
> Because the LAZY and STRICT have their own advantages and disadvantages.
>
> Should I say: If unsure, keep the default。

Maybe. So you could put this in the help for the choice, * above, and 
remove the advisory on IOMMU_DEFAULT_PASSTHROUGH.

However the maintainer may have a different view.

Thanks,
John

>
>>
>>> +
>>> +config IOMMU_DEFAULT_STRICT
>>> +    bool "strict"
>>> +    help
>>> +      For every IOMMU DMA unmap operation, the flush operation of IOTLB and
>>> +      the free operation of IOVA are guaranteed to be done in the unmap
>>> +      function.
>>> +
>>> +      This mode is safer than the two above, but it maybe slower in some
>>> +      high performace scenarios.
>>
>> and here?

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