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

[Lu Baolu <baolu.lu@linux.intel.com>]

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@linux.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