linux-kernel - Re: [PATCH 00/20] iommu: Refactoring domain allocation interface

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Date: Fri, 31 May 2024 14:00:34 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Yi Liu <yi.l.liu@...el.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Jason Gunthorpe <jgg@...pe.ca>, Kevin Tian <kevin.tian@...el.com>
Cc: baolu.lu@...ux.intel.com, David Airlie <airlied@...il.com>,
 Daniel Vetter <daniel@...ll.ch>, Kalle Valo <kvalo@...nel.org>,
 Bjorn Andersson <andersson@...nel.org>,
 Mathieu Poirier <mathieu.poirier@...aro.org>,
 Alex Williamson <alex.williamson@...hat.com>, mst@...hat.com,
 Jason Wang <jasowang@...hat.com>, Thierry Reding <thierry.reding@...il.com>,
 Jonathan Hunter <jonathanh@...dia.com>,
 Mikko Perttunen <mperttunen@...dia.com>, iommu@...ts.linux.dev,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/20] iommu: Refactoring domain allocation interface

On 5/31/24 11:16 AM, Yi Liu wrote:
> On 2024/5/29 20:02, Baolu Lu wrote:
>> On 2024/5/29 17:03, Yi Liu wrote:
>>> On 2024/5/29 13:32, Lu Baolu wrote:
>>>> The IOMMU subsystem has undergone some changes, including the removal
>>>> of iommu_ops from the bus structure. Consequently, the existing domain
>>>> allocation interface, which relies on a bus type argument, is no longer
>>>> relevant:
>>>>
>>>>      struct iommu_domain *iommu_domain_alloc(struct bus_type *bus)
>>>>
>>>> This series is designed to refactor the use of this interface. It
>>>> proposes two new interfaces to replace iommu_domain_alloc():
>>>>
>>>> - iommu_user_domain_alloc(): This interface is intended for allocating
>>>>    iommu domains managed by userspace for device passthrough scenarios,
>>>>    such as those used by iommufd, vfio, and vdpa. It clearly indicates
>>>>    that the domain is for user-managed device DMA.
>>>
>>> user paging domain? It looks to me user domain includes the nested 
>>> domains
>>> as well.
>>
>> Yes, nested domain is a user domain. The iommu driver should implement
>> iommu_ops->domain_alloc_user for nested domain allocation.
> 
> will it be more clear to name iommu_user_domain_alloc() be
> iommu_user_paging_domain_alloc() as it is mainly for paging domain
> allocation?

That might be better; let's wait and see if there's another option.

> 
>>>
>>>>    If an IOMMU driver does not implement iommu_ops->domain_alloc_user,
>>>>    this interface will rollback to the generic paging domain 
>>>> allocation.
>>>>
>>>> - iommu_paging_domain_alloc(): This interface is for allocating iommu
>>>>    domains managed by kernel drivers for kernel DMA purposes. It 
>>>> takes a
>>>>    device pointer as a parameter, which better reflects the current
>>>>    design of the IOMMU subsystem.
>>>>
>>>> The majority of device drivers currently using iommu_domain_alloc() do
>>>> so to allocate a domain for a specific device and then attach that
>>>> domain to the device. These cases can be straightforwardly migrated to
>>>> the new interfaces.
>>>>
>>>> However, there are some drivers with more complex use cases that do
>>>> not fit neatly into this new scheme. For example:
>>>>
>>>> $ git grep "= iommu_domain_alloc"
>>>> arch/arm/mm/dma-mapping.c:      mapping->domain = 
>>>> iommu_domain_alloc(bus);
>>>> drivers/gpu/drm/rockchip/rockchip_drm_drv.c:    private->domain = 
>>>> iommu_domain_alloc(private->iommu_dev->bus);
>>>> drivers/gpu/drm/tegra/drm.c:            tegra->domain = 
>>>> iommu_domain_alloc(&platform_bus_type);
>>>> drivers/infiniband/hw/usnic/usnic_uiom.c:       pd->domain = domain 
>>>> = iommu_domain_alloc(dev->bus);
>>>>
>>>> This series leave those cases unchanged and keep iommu_domain_alloc()
>>>> for their usage. But new drivers should not use it anymore.
>>>
>>> does it mean there is still domains allocated via iommu_domain_alloc()
>>> on VT-d platform?
>>
>> I think the drivers mentioned above do not run on x86 platforms, or do
>> they?
> 
> cool. BTW. I know out-of-tree drivers are not counted in upstream review.
> Just out of curious, is there a formal way to let such drivers know it is
> no longer allowed to use iommu_domain_alloc() on VT-d?

As Robin suggested, we should try to remove iommu_domain_alloc() from
the tree in this series.

Best regards,
baolu