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

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Date:   Fri, 31 May 2019 18:03:16 +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 2019/5/30 20:20, John Garry wrote:
> On 30/05/2019 04:48, Zhen Lei wrote:
>> First, add build option IOMMU_DEFAULT_{LAZY|STRICT}, so that we have the
>> opportunity to set {lazy|strict} mode as default at build time. Then put
>> the three config options in an choice, make people can only choose one of
>> the three at a time.
>>
> 
> Since this was not picked up, but modulo (somtimes same) comments below:
> 
> Reviewed-by: John Garry <john.garry@...wei.com>
> 
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> ---
>>  drivers/iommu/Kconfig | 42 +++++++++++++++++++++++++++++++++++-------
>>  drivers/iommu/iommu.c |  3 ++-
>>  2 files changed, 37 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>> index 83664db5221df02..d6a1a45f80ffbf5 100644
>> --- a/drivers/iommu/Kconfig
>> +++ b/drivers/iommu/Kconfig
>> @@ -75,17 +75,45 @@ config IOMMU_DEBUGFS
>>        debug/iommu directory, and then populate a subdirectory with
>>        entries as required.
>>
>> -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。

> 
>> +
>> +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?
> 
>> +
>> +endchoice
>> +
>>  config OF_IOMMU
>>         def_bool y
>>         depends on OF && IOMMU_API
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 67ee6623f9b2a4d..56bce221285b15f 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -43,7 +43,8 @@
>>  #else
>>  static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_DMA;
>>  #endif
>> -static bool iommu_dma_strict __read_mostly = true;
>> +static bool iommu_dma_strict __read_mostly =
>> +            IS_ENABLED(CONFIG_IOMMU_DEFAULT_STRICT);
>>
>>  struct iommu_group {
>>      struct kobject kobj;
>>
> 
> 
> 
> .
>