linux-kernel - Re: [PATCH 4/4] iommu/vt-d: Remove lazy allocation of domains

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Date:   Sun, 10 Mar 2019 10:51:59 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     James Sewart <jamessewart@...sta.com>
Cc:     baolu.lu@...ux.intel.com, iommu@...ts.linux-foundation.org,
        Tom Murphy <tmurphy@...sta.com>,
        Dmitry Safonov <dima@...sta.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] iommu/vt-d: Remove lazy allocation of domains

Hi,

On 3/9/19 7:49 PM, James Sewart wrote:
> Hey Lu,
> 
>> On 9 Mar 2019, at 01:53, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>>
>> Hi James,
>>
>> On 3/9/19 12:57 AM, James Sewart wrote:
>>> Hey Lu,
>>>> On 8 Mar 2019, at 03:09, Lu Baolu<baolu.lu@...ux.intel.com>  wrote:
>>>>>>
>>>>>> Do you mind if I work on top of your patches for further cleanups and
>>>>>> sign off a v2 together with you?
>>>>> Sure, sounds good. I’ll fixup patch 3 and have a go at integrating
>>>>> iommu_prepare_isa into get_resv_regions. This should make the initial
>>>>> domain logic here quite concise.
>>>> Here attached three extra patches which I think should be added before
>>>> PATCH 3/4, and some further cleanup changes which you can merge with
>>>> PATCH 4/4.
>>>>
>>>> ----------------
>>>>
>>>> 0006-iommu-Add-ops-entry-for-vendor-specific-default-doma.patch
>>>> 0007-iommu-vt-d-Add-is_identity_map-ops-entry.patch
>>>>
>>>> These two patches aim to add a generic method for vendor specific iommu
>>>> drivers to specify the type of the default domain for a device. Intel
>>>> iommu driver will register an ops for this since it already has its own
>>>> identity map adjudicator for a long time.
>>> This seems like a good idea, but as domain alloc is only called for the
>>> default domain on first device attached to a group, we may miss checking
>>> whether a device added later should have an identity domain. Should there
>>> be paths to downgrade a groups domain if one of the devices requires one?
>>
>> Good catch!
>>
>> This is supposed to be handled in iommu_no_mapping(). But, obviously
>> current code sticks to lazy domain allocation. I'm not sure whether
>> there are any real such cases, but we should handle it in a clean way.
>> My idea is that we could downgrade to multiple domains per group (just
>> like what we have now) in this case and print a kernel message for this.
> 
> I think if a device requires an identity domain, then it should ignore
> attempts to attach to something else. A print to warn a user about this
> would be a good idea.
> 
> I figure during attach: if iommu_no_mapping() then attach to si_domain and
> print, else continue with the given domain.

Yes, agree.

Best regards,
Lu Baolu