linux-kernel - Re: [PATCH v1 1/8] iommu: Add iommu_group_replace

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Date:   Mon, 14 Feb 2022 14:10:19 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, kvm@...r.kernel.org,
        rafael@...nel.org, David Airlie <airlied@...ux.ie>,
        linux-pci@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Will Deacon <will@...nel.org>,
        Stuart Yoder <stuyoder@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, Li Yang <leoyang.li@....com>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()

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.