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

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Message-ID: <yq5aecvktj97.fsf@kernel.org>
Date: Mon, 16 Jun 2025 09:45:00 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: "Tian, Kevin" <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: "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

"Tian, Kevin" <kevin.tian@...el.com> writes:

>> From: Jason Gunthorpe <jgg@...pe.ca>
>> Sent: Friday, June 13, 2025 1:27 AM
>> 
>> 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/53025c827c44d68edb6469bfd940a8e8bc6147a5.1
>> 729897278.git.nicolinc@...dia.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?
>> 
>
> yeah that seems to be the option if the said life-cycle dependency
> cannot be removed...
>
> conceptually it's still a bit unclean as the user needs to know that
> the vdevice object is special after idevice is unbound i.e. it can only
> be destroyed instead of supporting any other kind of operations.
>
> hmm if the user needs to build certain knowledge anyway can we 
> go one step further to state that the vdevice will be destroyed
> automatically once its idevice is unbound so the user shouldn't
> attempt to explicitly destroy it again after unbind?

That is what this patch is does. ie, it automatically destroy the
vdevice while unbinding the idevice.

-aneesh