linux-kernel - Re: [PATCH v2 3/7] iommu/vt-d: Expose ISA direct mapping region via iommu_get_resv

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Date:   Wed, 10 Apr 2019 13:22:30 +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 v2 3/7] iommu/vt-d: Expose ISA direct mapping region via
 iommu_get_resv_regions

Hi James,

On 4/6/19 2:02 AM, James Sewart wrote:
> Hey Lu,
> 
> My bad, did some debugging on my end. The issue was swapping out
> find_domain for iommu_get_domain_for_dev. It seems in some situations the
> domain is not attached to the group but the device is expected to have the
> domain still stored in its archdata.
> 
> I’ve attached the final patch with find_domain unremoved which seems to
> work in my testing.
> 

Just looked into your v3 patch set and some thoughts from my end posted
here just for your information.

Let me post the problems we want to address.

1. When allocating a new group for a device, how should we determine the
type of the default domain?

2. If we need to put a device into an existing group which uses a
different type of domain from what the device desires to use, we might
break the functionality of the device.

My new thought is letting the iommu generic code to determine the
default domain type (hence my proposed vendor specific default domain
type patches could be dropped).

If the default domain type is dynamical mapping, and later in 
iommu_no_mapping(), we determines that we must use an identity domain,
we then call iommu_request_dm_for_dev(dev).

If the default domain type is identity mapping, and later in
iommu_no_mapping(), we determined that we must use a dynamical domain,
we then call iommu_request_dma_domain_for_dev(dev).

We already have iommu_request_dm_for_dev() in iommu.c. We only need to
implement iommu_request_dma_domain_for_dev().

With this done, your patch titled "Create an IOMMU group for devices
that require an identity map" could also be dropped.

Any thoughts?

> Cheers,
> James.

Best regards,
Lu Baolu