linux-kernel - Re: [RFC PATCH 3/4] iommu: Preallocate iommu group when probing devices

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Date:   Sun, 19 Jan 2020 14:29:17 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     baolu.lu@...ux.intel.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, ashok.raj@...el.com,
        jacob.jun.pan@...el.com, kevin.tian@...el.com,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] iommu: Preallocate iommu group when probing
 devices

Hi Joerg,

On 1/17/20 6:21 PM, Joerg Roedel wrote:
> On Wed, Jan 01, 2020 at 01:26:47PM +0800, Lu Baolu wrote:
>> This splits iommu group allocation from adding devices. This makes
>> it possible to determine the default domain type for each group as
>> all devices belonging to the group have been determined.
> 
> I think its better to keep group allocation as it is and just defer
> default domain allocation after each device is in its group. But take

I tried defering default domain allocation, but it seems not possible.

The call path of adding devices into their groups:

iommu_probe_device
-> ops->add_device(dev)
    -> (iommu vendor driver) iommu_group_get_for_dev(dev)

After doing this, the vendor driver will get the default domain and
apply dma_ops according to the domain type. If we defer the domain
allocation, they will get a NULL default domain and cause panic in
the vendor driver.

Any suggestions?

Best regards,
baolu

> care about the device hotplug path which might add new devices to groups
> which already have a default domain, or add new groups that might need a
> default domain too.