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

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Date:   Thu, 27 Oct 2016 18:01:51 +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 10/27/2016 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?

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
> 
>