Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()

From: Robin Murphy <robin.murphy@arm.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Lu Baolu <baolu.lu@linux.intel.com>,
	Joerg Roedel <joro@8bytes.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	kvm@vger.kernel.org, rafael@kernel.org,
	David Airlie <airlied@linux.ie>,
	linux-pci@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Dmitry Osipenko <digetx@gmail.com>, Will Deacon <will@kernel.org>,
	Stuart Yoder <stuyoder@gmail.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Cornelia Huck <cohuck@redhat.com>,
	linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()
Date: Mon, 14 Feb 2022 14:10:19 +0000	[thread overview]
Message-ID: <1347f0ef-e046-1332-32f0-07347cc2079c@arm.com> (raw)
In-Reply-To: <20220214124518.GU4160@nvidia.com>

On 2022-02-14 12:45, Jason Gunthorpe wrote:
> On Mon, Feb 14, 2022 at 12:09:36PM +0000, Robin Murphy wrote:
>> On 2022-01-06 02:20, Lu Baolu wrote:
>>> Expose an interface to replace the domain of an iommu group for frameworks
>>> like vfio which claims the ownership of the whole iommu group.
>>
>> But if the underlying point is the new expectation that
>> iommu_{attach,detach}_device() operate on the device's whole group where
>> relevant, why should we invent some special mechanism for VFIO to be
>> needlessly inconsistent?
>>
>> I said before that it's trivial for VFIO to resolve a suitable device if it
>> needs to; by now I've actually written the patch ;)
>>
>> https://gitlab.arm.com/linux-arm/linux-rm/-/commit/9f37d8c17c9b606abc96e1f1001c0b97c8b93ed5
> 
> Er, how does locking work there? What keeps busdev from being
> concurrently unplugged?

Same thing that prevents the bus pointer from suddenly becoming invalid 
in the current code, I guess :)

But yes, holding a group reference alone can't prevent the group itself 
from changing, and the finer points of locking still need working out - 
there's a reason you got a link to a WIP branch in my tree rather than a 
proper patch in your inbox (TBH at the moment that one represents about 
a 5:1 ratio of time spent on the reasoning behind the commit message vs. 
the implementation itself).

> How can iommu_group_get() be safely called on
> this pointer?

VFIO hardly needs to retrieve the iommu_group from a device which it 
derived from the iommu_group it holds in the first place. What matters 
is being able to call *other* device-based IOMMU API interfaces in the 
long term. And once a robust solution for that is in place, it should 
inevitably work for a device-based attach interface too.

> All of the above only works normally inside a probe/remove context
> where the driver core is blocking concurrent unplug and descruction.
> 
> I think I said this last time you brought it up that lifetime was the
> challenge with this idea.

Indeed, but it's a challenge that needs tackling, because the bus-based 
interfaces need to go away. So either we figure it out now and let this 
attach interface rework benefit immediately, or I spend three times as 
long solving it on my own and end up deleting 
iommu_group_replace_domain() in about 6 months' time anyway.

Thanks,
Robin.