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

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Date:   Thu, 13 Jun 2019 16:30:08 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     John Garry <john.garry@...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



On 2019/5/31 18:42, John Garry wrote:
> 
>>>> -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.

OK, I'll revise it according to this idea in v9.

> 
> 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?
> 
> 
> .
>