Hi Joerg, On 11/19/21 11:06 PM, Jörg Rödel wrote:
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.
Thanks for the explanation. It's very helpful. There seems to be a lot of discussions around this, but I didn't see any meaningful reasons to break the assumption of "all devices in a group being in a same address space". Best regards, baolu
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
