linux-kernel - Re: [PATCH v5 09/14] PCI: portdrv: Suppress kernel DMA ownership auto-claiming

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Message-ID: <20220104192614.GL2328285@nvidia.com>
Date:   Tue, 4 Jan 2022 15:26:14 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Dan Williams <dan.j.williams@...el.com>, rafael@...nel.org,
        Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Stuart Yoder <stuyoder@...il.com>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Li Yang <leoyang.li@....com>,
        Dmitry Osipenko <digetx@...il.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 09/14] PCI: portdrv: Suppress kernel DMA ownership
 auto-claiming

On Tue, Jan 04, 2022 at 11:06:31AM -0600, Bjorn Helgaas wrote:

> > The existing vfio framework allows the portdrv driver to be bound
> > to the bridge while its downstream devices are assigned to user space.
> 
> I.e., the existing VFIO framework allows a switch to be in the same
> IOMMU group as the devices below it, even though the switch has a
> kernel driver and the other devices may have userspace drivers?

Yes, this patch exists to maintain current VFIO behavior which has this
same check.

I belive the basis for VFIO doing this is that the these devices
cannot do DMA, so don't care about the DMA API or the group->domain,
and do not expose MMIO memory so do not care about the P2P attack.

A comment in the code to this effect would be good, IMHO.

Jason