linux-kernel - Re: [PATCH 01/11] iommu: Add device dma ownership set/release interfaces

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Date:   Fri, 19 Nov 2021 08:56:18 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Cornelia Huck <cohuck@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 01/11] iommu: Add device dma ownership set/release
 interfaces

On Fri, Nov 19, 2021 at 05:44:35AM +0000, Tian, Kevin wrote:

> Well, the difference is just in literal. I don't know the background
> why the existing iommu_attach_device() users want to do it this
> way. But given the condition in iommu_attach_device() it could
> in theory imply some unknown hardware-level side effect which 
> may break the desired functionality once the group size grows 
> beyond singleton. Is it a real case? I don't know...

No, this must not be.

Any device can have VFIO attached to it, and VFIO does the equivalent
of iommu_attach_device(). We cannot have "unknown hardware-level side
effects" to changing the domain's of multi device groups and also have
working VFIO.

If these HW problems exist they are a seperate issue and the iommu
core should flag such groups as being unable to change their domains
at all.

Jason