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 18:20:16 +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 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.



> - 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