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

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Message-ID: <7af4fee2-1b37-4eb8-9d03-8b1a402ec00b@intel.com>
Date: Fri, 31 May 2024 11:16:03 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.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: 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 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?

>>
>>>    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?

-- 
Regards,
Yi Liu