linux-kernel - Re: [PATCH v6 01/11] iommu: Add dma ownership management interfaces

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Date:   Wed, 23 Feb 2022 14:32:48 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Stuart Yoder <stuyoder@...il.com>, 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>, Ashok Raj <ashok.raj@...el.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        kvm@...r.kernel.org, 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 v6 01/11] iommu: Add dma ownership management interfaces

On Wed, Feb 23, 2022 at 06:20:36PM +0000, Robin Murphy wrote:
> On 2022-02-23 18:02, Jason Gunthorpe via iommu wrote:
> > On Wed, Feb 23, 2022 at 06:00:06PM +0000, Robin Murphy wrote:
> > 
> > > ...and equivalently just set owner_cnt directly to 0 here. I don't see a
> > > realistic use-case for any driver to claim the same group more than once,
> > > and allowing it in the API just feels like opening up various potential
> > > corners for things to get out of sync.
> > 
> > I am Ok if we toss it out to get this merged, as there is no in-kernel
> > user right now.
> > 
> > Something will have to come back for iommufd, but we can look at what
> > is best suited then.
> 
> If iommufd plans to be too dumb to keep track of whether it already owns a
> given group or not, I can't see it dealing with attaching that group to a
> single domain no more than once, either ;)

Indeed, this is why I'd like to use the device API :)

Jason