RE: [PATCH v2 1/3] iommufd: Add devices_users to track the hw_pagetable usage by device

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Sunday, January 29, 2023 5:18 AM
> 
> From: Yi Liu <yi.l.liu@xxxxxxxxx>
> 
> Currently, hw_pagetable tracks the attached devices using a device list.
> When attaching the first device to the kernel-managed hw_pagetable, it
> should be linked to IOAS. When detaching the last device from this hwpt,
> the link with IOAS should be removed too. And this first-or-last device
> check is done with list_empty(hwpt->devices).
> 
> However, with a nested configuration, when a device is attached to the
> user-managed stage-1 hw_pagetable, it will be added to this user-managed
> hwpt's device list instead of the kernel-managed stage-2 hwpt's one. And
> this breaks the logic for a kernel-managed hw_pagetable link/disconnect
> to/from IOAS/IOPT. e.g. the stage-2 hw_pagetable would be linked to IOAS
> multiple times if multiple device is attached, but it will become empty
> as soon as one device detached.
> 
> Add a devices_users in struct iommufd_hw_pagetable to track the users of

device_users

> hw_pagetable by the attached devices. Make this field as a pointer, only
> allocate for a stage-2 hw_pagetable. A stage-1 hw_pagetable should reuse
> the stage-2 hw_pagetable's devices_users, because when a device attaches
> to a stage-1 hw_pagetable, linking the stage-2 hwpt to the IOAS is still
> required. So, with a nested configuration, increase the devices_users on
> the stage-2 (parent) hwpt, no matter a device is attached to the stage-1
> or the stage-2 hwpt.

Above is very confusing w/o seeing the full series of nesting support.

As a preparatory step this should focus on existing code and what this
series tries to achieve. e.g. I'd not make device_users a pointer here.
Do that incrementally when the nesting support comes.