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

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Message-ID: <62ade373-6edc-c7f3-c205-200cf4fd211f@nvidia.com>
Date:   Tue, 1 Nov 2016 19:31:42 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>, <alex.williamson@...hat.com>,
        <pbonzini@...hat.com>, <kraxel@...hat.com>, <cjia@...dia.com>
CC:     <jike.song@...el.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 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.

Thanks,
Kirti