linux-kernel - Re: [RFC PATCH] iommufd: Destroy vdevice on device unbind

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Message-ID: <20250612172645.GA1011960@ziepe.ca>
Date: Thu, 12 Jun 2025 14:26:45 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC PATCH] iommufd: Destroy vdevice on device unbind

On Thu, Jun 12, 2025 at 08:05:37AM +0000, Tian, Kevin wrote:
> The initial v5 patch [1] from Nicolin was similar to what this
> patch does. Jason explained [2] why it's unsafe to destroy "userspace
> created" objects behind the scene. And a general rule in iommufd is
> to not take long term references on kernel owned objects.
> 
> [1] https://lore.kernel.org/all/53025c827c44d68edb6469bfd940a8e8bc6147a5.1729897278.git.nicolinc@nvidia.com/
> [2] https://lore.kernel.org/all/20241029184801.GW6956@nvidia.com/

Yes, we have a problem here where we both can't let VFIO go away while
the vdevice is present nor can we let the vdevice be fully deleted.

At that point it wasn't such a big deal, but the new stuff seems to
make vdevice more complicated that it cannot out live the idevice.

Probably the answer is to tombstone the vdevice in the xarray so the
ID is still there and userspace can still destroy it while destroying
everything linked to it?

Jason