linux-kernel - Re: [PATCH v10 1/3] iommu: Enhance IOMMU default DMA mode build options

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Date:   Fri, 4 Jun 2021 09:19:57 +0100
From:   John Garry <john.garry@...wei.com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
        <joro@...tes.org>, <will@...nel.org>, <dwmw2@...radead.org>,
        <baolu.lu@...ux.intel.com>, <robin.murphy@....com>
CC:     <linux-kernel@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <linuxarm@...wei.com>, <chenxiang66@...ilicon.com>
Subject: Re: [PATCH v10 1/3] iommu: Enhance IOMMU default DMA mode build
 options


>>   
>>   	  If unsure, say N here.
>>   
>> +choice
>> +	prompt "IOMMU default DMA mode"
>> +	depends on IOMMU_API
> 
> config INTEL_IOMMU
>          depends on PCI_MSI && ACPI && (X86 || IA64)
> 
> config AMD_IOMMU
>          depends on X86_64 && PCI && ACPI && HAVE_CMPXCHG_DOUBLE
> 
> config ARM_SMMU_V3
>          depends on ARM64
> 
> "depends on ARM_SMMU_V3 || INTEL_IOMMU || AMD_IOMMU" may need to be added, because it doesn't work on other platforms.
> 
> or "depends on X86 || IA64 || X86_64 || ARM64"

I suppose so. But I don't think that any other archs use the value from 
iommu_dma_strict anyway.

> 
>> +
>> +	default IOMMU_DEFAULT_STRICT
>> +	help
>> +	  This option allows an IOMMU DMA mode to be chosen at build time, to
>> +	  override the default DMA mode of each ARCH, removing the need to
>> +	  pass in kernel parameters through command line. It is still possible
>> +	  to provide ARCH-specific or common boot options to override this
>> +	  option.
>> +
>> +	  If unsure, keep the default.
>> +
>> +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.
>> +
>> +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.
>> +
>> +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 966426a96520..177b0dafc535 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -29,7 +29,8 @@ static struct kset *iommu_group_kset;
>>   static DEFINE_IDA(iommu_group_ida);
>>   
>>   static unsigned int iommu_def_domain_type __read_mostly;
>> -static bool iommu_dma_strict __read_mostly = true;
>> +static bool iommu_dma_strict __read_mostly =
>> +			IS_ENABLED(CONFIG_IOMMU_DEFAULT_STRICT);
> 
> Currently, a maximum of 100 columns are allowed in a row.

Sure, but some people still prefer limiting to 80, and codingstyle
doc mentions a preference for 80.

But if you really think I should change, then that's ok.

Thanks,
John