linux-kernel - Re: [RFC PATCH 0/4] iommu: Per-group default domain type

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Date:   Wed, 22 Jan 2020 12:58:42 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     John Garry <john.garry@...wei.com>, Joerg Roedel <joro@...tes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     baolu.lu@...ux.intel.com, kevin.tian@...el.com,
        ashok.raj@...el.com, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jacob.jun.pan@...el.com,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Subject: Re: [RFC PATCH 0/4] iommu: Per-group default domain type

Hi,

On 1/21/20 6:14 PM, John Garry wrote:
> On 21/01/2020 00:43, Lu Baolu wrote:
>>>> An IOMMU group represents the smallest set of devices that are 
>>>> considered
>>>> to be isolated. All devices belonging to an IOMMU group share a default
>>>> domain for DMA APIs. There are two types of default domain: 
>>>> IOMMU_DOMAIN_DMA
>>>> and IOMMU_DOMAIN_IDENTITY. The former means IOMMU translation, while 
>>>> the
>>>> latter means IOMMU by-pass.
>>>>
>>>> Currently, the default domain type for the IOMMU groups is determined
>>>> globally. All IOMMU groups use a single default domain type. The global
>>>> default domain type can be adjusted by kernel build configuration or
>>>> kernel parameters.
>>>>
>>>> More and more users are looking forward to a fine grained default 
>>>> domain
>>>> type. For example, with the global default domain type set to 
>>>> translation,
>>>> the OEM verndors or end users might want some trusted and fast-speed 
>>>> devices
>>>> to bypass IOMMU for performance gains. On the other hand, with global
>>>> default domain type set to by-pass, some devices with limited system
>>>> memory addressing capability might want IOMMU translation to remove the
>>>> bounce buffer overhead.
>>>
>>> Hi Lu Baolu,
>>>
>>> Do you think that it would be a more common usecase to want 
>>> kernel-managed devices to be passthrough for performance reasons and 
>>> some select devices to be in DMA domain, like those with limited 
>>> address cap or whose drivers request huge amounts of memory?
>>>
>>> I just think it would be more manageable to set kernel commandline 
>>> parameters for this, i.e. those select few which want DMA domain.
>>>
> 
> Hi Baolu,
> 
>>
>> It's just two sides of a coin. Currently, iommu subsystem make DMA
>> domain by default, that's the reason why I selected to let user set
>> which devices are willing to use identity domains.
>>
> 
> OK, understood.
> 
> There was an alternate solution here which would allow per-group type to 
> be updated via sysfs:
> 
> https://lore.kernel.org/linux-iommu/cover.1566353521.git.sai.praneeth.prakhya@intel.com/ 
> 

Yes. My patch set just tries to do this statically during boot time.

> 
> Any idea what happened to that?

No idea. Sai might have more information. :-)

Best regards,
baolu