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

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Message-ID: <de8f306a-f7f9-f1f4-a8f6-97897b4c78e7@linux.intel.com>
Date:   Sat, 9 Mar 2019 09:53:32 +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 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.

Or any other thoughts?

Best regards,
Lu Baolu