linux-kernel - Re: [PATCH 01/11] iommu: Add device dma ownership set/release interfaces

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Date:   Thu, 18 Nov 2021 09:12:41 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>, kvm@...r.kernel.org,
        rafael@...nel.org, linux-pci@...r.kernel.org,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 01/11] iommu: Add device dma ownership set/release
 interfaces

On 11/17/21 9:35 PM, Jason Gunthorpe wrote:
> On Wed, Nov 17, 2021 at 01:22:19PM +0800, Lu Baolu wrote:
>> Hi Jason,
>>
>> On 11/16/21 9:46 PM, Jason Gunthorpe wrote:
>>> On Tue, Nov 16, 2021 at 09:57:30AM +0800, Lu Baolu wrote:
>>>> Hi Christoph,
>>>>
>>>> On 11/15/21 9:14 PM, Christoph Hellwig wrote:
>>>>> On Mon, Nov 15, 2021 at 10:05:42AM +0800, Lu Baolu wrote:
>>>>>> +enum iommu_dma_owner {
>>>>>> +	DMA_OWNER_NONE,
>>>>>> +	DMA_OWNER_KERNEL,
>>>>>> +	DMA_OWNER_USER,
>>>>>> +};
>>>>>> +
>>>>>
>>>>>> +	enum iommu_dma_owner dma_owner;
>>>>>> +	refcount_t owner_cnt;
>>>>>> +	struct file *owner_user_file;
>>>>>
>>>>> I'd just overload the ownership into owner_user_file,
>>>>>
>>>>>     NULL			-> no owner
>>>>>     (struct file *)1UL)	-> kernel
>>>>>     real pointer		-> user
>>>>>
>>>>> Which could simplify a lot of the code dealing with the owner.
>>>>>
>>>>
>>>> Yeah! Sounds reasonable. I will make this in the next version.
>>>
>>> It would be good to figure out how to make iommu_attach_device()
>>> enforce no other driver binding as a kernel user without a file *, as
>>> Robin pointed to, before optimizing this.
>>>
>>> This fixes an existing bug where iommu_attach_device() only checks the
>>> group size and is vunerable to a hot plug increasing the group size
>>> after it returns. That check should be replaced by this series's logic
>>> instead.
>>
>> As my my understanding, the essence of this problem is that only the
>> user owner of the iommu_group could attach an UNMANAGED domain to it.
>> If I understand it right, how about introducing a new interface to
>> allocate a user managed domain and storing the user file pointer in it.
> 
> For iommu_attach_device() the semantic is simple non-sharing, so there
> is no need for the file * at all, it can just be NULL.

The file * being NULL means the device is only owned by the kernel
driver. Perhaps we can check this pointer in iommu_attach_device() to
avoid using it for user domain attachment.

> 
>> Does above help here?
> 
> No, iommu_attach_device() is kernel only and should not interact with
> userspace.

The existing iommu_attach_device() allows only for singleton group. As
we have added group ownership attribute, we can enforce this interface
only for kernel domain usage.

> 
> I'm also going to see if I can learn what Tegra is doing with
> iommu_attach_group()

Okay! Thank you!

> 
> Jason
> 

Best regards,
baolu