linux-kernel - Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for mediated devices

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Message-ID: <5819DA45.9000200@intel.com>
Date:   Wed, 02 Nov 2016 20:21:25 +0800
From:   Jike Song <jike.song@...el.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>
CC:     Kirti Wankhede <kwankhede@...dia.com>, alex.williamson@...hat.com,
        pbonzini@...hat.com, kraxel@...hat.com, cjia@...dia.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        kevin.tian@...el.com, qemu-devel@...gnu.org,
        bjsdjshi@...ux.vnet.ibm.com
Subject: Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for mediated
 devices

On 11/02/2016 12:09 PM, Alexey Kardashevskiy wrote:
> On 02/11/16 14:29, Kirti Wankhede wrote:
>>
>>
>> On 11/2/2016 6:54 AM, Alexey Kardashevskiy wrote:
>>> On 02/11/16 01:01, Kirti Wankhede wrote:
>>>>
>>>>
>>>> On 10/28/2016 7:48 AM, Alexey Kardashevskiy wrote:
>>>>> On 27/10/16 23:31, Kirti Wankhede wrote:
>>>>>>
>>>>>>
>>>>>> On 10/27/2016 12:50 PM, Alexey Kardashevskiy wrote:
>>>>>>> On 18/10/16 08:22, Kirti Wankhede wrote:
>>>>>>>> VFIO IOMMU drivers are designed for the devices which are IOMMU capable.
>>>>>>>> Mediated device only uses IOMMU APIs, the underlying hardware can be
>>>>>>>> managed by an IOMMU domain.
>>>>>>>>
>>>>>>>> Aim of this change is:
>>>>>>>> - To use most of the code of TYPE1 IOMMU driver for mediated devices
>>>>>>>> - To support direct assigned device and mediated device in single module
>>>>>>>>
>>>>>>>> Added two new callback functions to struct vfio_iommu_driver_ops. Backend
>>>>>>>> IOMMU module that supports pining and unpinning pages for mdev devices
>>>>>>>> should provide these functions.
>>>>>>>> Added APIs for pining and unpining pages to VFIO module. These calls back
>>>>>>>> into backend iommu module to actually pin and unpin pages.
>>>>>>>>
>>>>>>>> This change adds pin and unpin support for mediated device to TYPE1 IOMMU
>>>>>>>> backend module. More details:
>>>>>>>> - When iommu_group of mediated devices is attached, task structure is
>>>>>>>>   cached which is used later to pin pages and page accounting.
>>>>>>>
>>>>>>>
>>>>>>> For SPAPR TCE IOMMU driver, I ended up caching mm_struct with
>>>>>>> atomic_inc(&container->mm->mm_count) (patches are on the way) instead of
>>>>>>> using @current or task as the process might be gone while VFIO container is
>>>>>>> still alive and @mm might be needed to do proper cleanup; this might not be
>>>>>>> an issue with this patchset now but still you seem to only use @mm from
>>>>>>> task_struct.
>>>>>>>
>>>>>>
>>>>>> Consider the example of QEMU process which creates VFIO container, QEMU
>>>>>> in its teardown path would release the container. How could container be
>>>>>> alive when process is gone?
>>>>>
>>>>> do_exit() in kernel/exit.c calls exit_mm() (which sets NULL to tsk->mm)
>>>>> first, and then releases open files by calling  exit_files(). So
>>>>> container's release() does not have current->mm.
>>>>>
>>>>
>>>> Incrementing usage count (get_task_struct()) while saving task structure
>>>> and decementing it (put_task_struct()) from release() should  work here.
>>>> Updating the patch.
>>>
>>> I cannot see how the task->usage counter prevents do_exit() from performing
>>> the exit, can you?
>>>
>>
>> It will not prevent exit from do_exit(), but that will make sure that we
>> don't have stale pointer of task structure. Then we can check whether
>> the task is alive and get mm pointer in teardown path as below:
> 
> 
> Or you could just reference and use @mm as KVM and others do. Or there is
> anything else you need from @current than just @mm?
> 

I agree. If @mm is the only thing needed, there is really no reason to
refer to the @task :-)

--
Thanks,
Jike