linux-kernel - Re: [PATCH v2 1/3] iommufd: Add devices_users to track the hw

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Date:   Mon, 30 Jan 2023 11:02:25 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     kevin.tian@...el.com, yi.l.liu@...el.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] iommufd: Add devices_users to track the
 hw_pagetable usage by device

On Sat, Jan 28, 2023 at 01:18:09PM -0800, Nicolin Chen wrote:
> From: Yi Liu <yi.l.liu@...el.com>
> 
> 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