Re: [PATCH 01/11] iommu: Add device dma ownership set/release interfaces

[Jörg Rödel]

On Fri, Nov 19, 2021 at 07:14:10PM +0800, Lu Baolu wrote:
> The singleton group requirement for iommu_attach/detach_device() was
> added by below commit:
> 
> commit 426a273834eae65abcfc7132a21a85b3151e0bce
> Author: Joerg Roedel <jroedel@xxxxxxx>
> Date:   Thu May 28 18:41:30 2015 +0200
> 
>     iommu: Limit iommu_attach/detach_device to devices with their own group
> 
>     This patch changes the behavior of the iommu_attach_device
>     and iommu_detach_device functions. With this change these
>     functions only work on devices that have their own group.
>     For all other devices the iommu_group_attach/detach
>     functions must be used.
> 
>     Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> 
> Joerg,can you please shed some light on the background of this
> requirement? Does above idea of transition from singleton group
> to group with single driver bound make sense to you?

This change came to be because the iommu_attach/detach_device()
interface doesn't fit well into a world with iommu-groups. Devices
within a group are by definition not isolated between each other, so
they must all be in the same address space (== iommu_domain). So it
doesn't make sense to allow attaching a single device within a group to
a different iommu_domain.

I know that in theory it is safe to allow devices within a group to be
in different domains because there iommu-groups catch multiple
non-isolation cases:

	1) Devices behind a non-ACS capable bridge or multiple functions
	   of a PCI device. Here it is safe to put the devices into
	   different iommu-domains as long as all affected devices are
	   controlled by the same owner.

	2) Devices which share a single request-id and can't be
	   differentiated by the IOMMU hardware. These always need to be
	   in the same iommu_domain.

To lift the single-domain-per-group requirement the iommu core code
needs to learn the difference between the two cases above.

Regards,

	Joerg