linux-kernel - Re: [PATCH v3 1/8] iommu: Add ops entry for supported default domain type

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Date:   Fri, 10 May 2019 13:29:30 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Robin Murphy <robin.murphy@....com>,
        Tom Murphy <tmurphy@...sta.com>
Cc:     baolu.lu@...ux.intel.com, "Tian, Kevin" <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jacob.jun.pan@...el.com,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v3 1/8] iommu: Add ops entry for supported default domain
 type

Hi Robin,

On 5/10/19 12:11 AM, Robin Murphy wrote:
> On 09/05/2019 03:30, Lu Baolu wrote:
>> Hi Robin,
>>
>> On 5/7/19 6:28 PM, Robin Murphy wrote:
>>> On 06/05/2019 16:32, Tom Murphy via iommu wrote:
>>>> The AMD driver already solves this problem and uses the generic
>>>> iommu_request_dm_for_dev function. It seems like both drivers have the
>>>> same problem and could use the same solution. Is there any reason we
>>>> can't have use the same solution for the intel and amd driver?
>>>>
>>>> Could we just  copy the implementation of the AMD driver? It would be
>>>> nice to have the same behavior across both drivers especially as we
>>>> move to make both drivers use more generic code.
>>>
>>> TBH I don't think the API really needs to be involved at all here. 
>>> Drivers can already not provide the requested default domain type if 
>>> they don't support it, so as long as the driver can ensure that the 
>>> device ends up with IOMMU or direct DMA ops as appropriate, I don't 
>>> see any great problem with drivers just returning a passthrough 
>>> domain when a DMA domain was requested, or vice versa (and logging a 
>>> message that the requested type was overridden). The only type that 
>>> we really do have to honour strictly is non-default (i.e. unmanaged) 
>>> domains.
>>
>> I agree with you that we only have to honor strictly the non-default
>> domains. But domain type saved in iommu_domain is consumed in iommu.c
>> and exposed to user through sysfs. It's not clean if the iommu driver
>> silently replace the default domain.
> 
> Right, I did get a bit ahead of myself there - the implicit step before 
> that is to fix default domain allocation so that the core actually 
> passes the relevant device which it has to hand, such that the IOMMU 
> drivers *can* make the right decision up-front.
> 

Yes, passing the relevant device when allocating the default domain so
that the IOMMU driver could make right decision seems to be a better
solution. Somebody can come up with a patch set to bring this up for
discussion. I won't include this in this patch set since it's not for
that purpose. I will follow the existing mechanism that is using on amd
and other iommu drivers.

Best regards,
Lu Baolu