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

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for mediated devices
Date: Wed, 2 Nov 2016 12:24:00 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

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?


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