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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 15:09:56 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

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?

> {
>         struct task_struct *task = domain->external_addr_space->task;
>         struct mm_struct *mm = NULL;
>         put_pfn(pfn, prot);
>         if (pid_alive(task))
>                 mm = get_task_mm(task);
>         if (mm) {
>                 if (do_accounting)
>                         vfio_lock_acct(task, -1);
>                 mmput(mm);
>         }
> }


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