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

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Message-ID: <f59af2fb-054a-1a30-6ce6-54a1f0a62dec@ozlabs.ru>
Date:   Fri, 28 Oct 2016 13:18:53 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Kirti Wankhede <kwankhede@...dia.com>, 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 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.



> 
> Kirti
> 
>>
>>
>>> - It keeps track of pinned pages for mediated domain. This data is used to
>>>   verify unpinning request and to unpin remaining pages while detaching, if
>>>   there are any.
>>> - Used existing mechanism for page accounting. If iommu capable domain
>>>   exist in the container then all pages are already pinned and accounted.
>>>   Accouting for mdev device is only done if there is no iommu capable
>>>   domain in the container.
>>> - Page accouting is updated on hot plug and unplug mdev device and pass
>>>   through device.
>>>
>>> Tested by assigning below combinations of devices to a single VM:
>>> - GPU pass through only
>>> - vGPU device only
>>> - One GPU pass through and one vGPU device
>>> - Linux VM hot plug and unplug vGPU device while GPU pass through device
>>>   exist
>>> - Linux VM hot plug and unplug GPU pass through device while vGPU device
>>>   exist
>>>
>>> Signed-off-by: Kirti Wankhede <kwankhede@...dia.com>
>>> Signed-off-by: Neo Jia <cjia@...dia.com>
>>> Change-Id: I295d6f0f2e0579b8d9882bfd8fd5a4194b97bd9a
>>
>>


-- 
Alexey