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

[Jason Gunthorpe]

On Sat, Jan 28, 2023 at 01:18:09PM -0800, Nicolin Chen wrote:
> 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.

Why this seems really weird to say.

The stage 2 is linked explicitly to the IOAS that drives it's
map/unmap

Why is there any implicit activity here? There should be no implicit
attach of the S2 to an IOAS ever.

Jason