linux-kernel - Re: [PATCH v11 1/3] iommu: Enhance IOMMU default DMA mode build options

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Date:   Wed, 9 Jun 2021 16:24:09 +0100
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>, <joro@...tes.org>,
        <will@...nel.org>, <dwmw2@...radead.org>,
        <baolu.lu@...ux.intel.com>
CC:     <rdunlap@...radead.org>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>, <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v11 1/3] iommu: Enhance IOMMU default DMA mode build
 options

On 09/06/2021 16:03, Robin Murphy wrote:
> On 2021-06-08 14:18, John Garry wrote:
>> From: Zhen Lei <thunder.leizhen@...wei.com>
>>
>> First, add build options IOMMU_DEFAULT_{LAZY|STRICT}, so that we have the
>> opportunity to set {lazy|strict} mode as default at build time. Then put
>> the two config options in a choice, as they are mutually exclusive.
>>
>> [jpg: Make choice between strict and lazy only (and not passthrough)]
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> Signed-off-by: John Garry <john.garry@...wei.com>
>> ---
>>   drivers/iommu/Kconfig | 35 +++++++++++++++++++++++++++++++++++
>>   drivers/iommu/iommu.c |  3 ++-
>>   2 files changed, 37 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>> index 1f111b399bca..369a3af9e5bf 100644
>> --- a/drivers/iommu/Kconfig
>> +++ b/drivers/iommu/Kconfig
>> @@ -90,6 +90,41 @@ config IOMMU_DEFAULT_PASSTHROUGH
>>         If unsure, say N here.
>> +choice
>> +    prompt "IOMMU default DMA mode"
>> +    depends on IOMMU_API
>> +    depends on X86 || IA64 || X86_64 || ARM || ARM64
> 
> Simply "depends on IOMMU_DMA" should suffice, since that's now the only 
> place where flush queues matter.

I suppose so.

Configs ARM64, AMD_IOMMU, and INTEL_IOMMU all select this.

> 
>> +
>> +    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 lazy mode, but it may be slower in some 
>> high
>> +      performance scenarios.
> 
> FWIW, as an end user who doesn't care much about the implementation 
> details I'd probably appreciate the actual implications being clearer, 
> i.e. what does "safer" mean in practice and what is it relative to?
> 

Fine, I can mention that lazy mode means that we have reduced device 
isolation and a dangerous window can be created between device driver 
DMA unmap and zapping the mapping in the IOMMU; however still much safer 
than passthrough/no IOMMU, which means no isolation at all.

Thanks,
John