linux-kernel - Re: [PATCH v2 1/1] iommu: Add config option to set lazy mode as default

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Message-ID: <5C9C6748.8050103@huawei.com>
Date:   Thu, 28 Mar 2019 14:18:48 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        John Garry <john.garry@...wei.com>,
        "Will Deacon" <will.deacon@....com>,
        Joerg Roedel <joro@...tes.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH v2 1/1] iommu: Add config option to set lazy mode as
 default



On 2019/3/28 3:18, Robin Murphy wrote:
> On 27/03/2019 15:00, Zhen Lei wrote:
>> This allows the default behaviour to be controlled by a kernel config
>> option instead of changing the command line for the kernel to include
>> "iommu.strict=0" on ARM64 where this is desired.
>>
>> This is similar to CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
>>
>> Note: At present, intel_iommu, amd_iommu and s390_iommu use lazy mode as
>> default, so there is no need to add code for them.
> 
> That seems a bit self-contradictory - if there's a real need for TLB flush behaviour to be statically configurable and not command-line-based then why should users of other architectures be exempt?
Yes，you're right. I will make this configuration to be effective for other architectures.

BTW, can you give some opinion about below patches?
https://patchwork.kernel.org/patch/10857601/

> 
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> ---
>>   drivers/iommu/Kconfig | 14 ++++++++++++++
>>   drivers/iommu/iommu.c |  5 +++++
>>   2 files changed, 19 insertions(+)
>>
>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>> index 6f07f3b21816c64..5daa110d0e83a07 100644
>> --- a/drivers/iommu/Kconfig
>> +++ b/drivers/iommu/Kconfig
>> @@ -85,6 +85,20 @@ config IOMMU_DEFAULT_PASSTHROUGH
>>
>>         If unsure, say N here.
>>
>> +config IOMMU_DMA_DEFAULT_LAZY_MODE
>> +    bool "IOMMU DMA use lazy mode to flush IOTLB and free IOVA"
>> +    depends on IOMMU_API
>> +    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. Removing the need to pass in kernel parameters through
>> +      command line. For example, iommu.strict=0 on ARM64. If this is
>> +      enabled, you can still disable with kernel parameters, such as
>> +      iommu.strict=1 depending on the architecture.
>> +
>> +      If unsure, say N here.
>> +
>>   config OF_IOMMU
>>          def_bool y
>>          depends on OF && IOMMU_API
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 33a982e33716369..5acb98e79b5b32d 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -43,7 +43,12 @@
>>   #else
>>   static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_DMA;
>>   #endif
>> +
>> +#ifdef CONFIG_IOMMU_DMA_DEFAULT_LAZY_MODE
>> +static bool iommu_dma_strict __read_mostly;
>> +#else
>>   static bool iommu_dma_strict __read_mostly = true;
>> +#endif
> 
> For a straightforward boolean, you can simply do:
> 
>     static bool foo = IS_ENABLED(CONFIG_FOO);
> 
> but that said, I'm still not particularly convinced that there are a significant number of users in a position to build and install a custom kernel but not edit /etc/default/grub, and who really value the combination of less performance than passthrough with less isolation than strict.
> 
> It's also not necessarily obvious to the user how this interacts with IOMMU_DEFAULT_PASSTHROUGH, so if we really do go down this route, maybe it would be better to refactor the whole lot into a single selection of something like IOMMU_DEFAULT_MODE anyway.
> 
> Robin.
> 
>>
>>   struct iommu_callback_data {
>>       const struct iommu_ops *ops;
>> -- 
>> 1.8.3
>>
>>
> 
> .
> 

-- 
Thanks!
BestRegards