linux-kernel - Re: [PATCH v8 1/7] iommu: enhance IOMMU default DMA mode build options

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Date:   Fri, 31 May 2019 11:42:45 +0100
From:   John Garry <john.garry@...wei.com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
        "Jean-Philippe Brucker" <jean-philippe.brucker@....com>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will.deacon@....com>,
        Joerg Roedel <joro@...tes.org>,
        "Jonathan Corbet" <corbet@....net>,
        linux-doc <linux-doc@...r.kernel.org>,
        Sebastian Ott <sebott@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        "Martin Schwidefsky" <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        x86 <x86@...nel.org>, linux-ia64 <linux-ia64@...r.kernel.org>
CC:     Hanjun Guo <guohanjun@...wei.com>, Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH v8 1/7] iommu: enhance IOMMU default DMA mode build
 options


>>> -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?