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

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Message-ID: <20220106183224.GA298861@bhelgaas>
Date:   Thu, 6 Jan 2022 12:32:24 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...dia.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 Thu, Jan 06, 2022 at 12:12:35PM +0800, Lu Baolu wrote:
> On 1/5/22 1:06 AM, Bjorn Helgaas wrote:
> > On Tue, Jan 04, 2022 at 09:56:39AM +0800, Lu Baolu wrote:
> > > If a switch lacks ACS P2P Request Redirect, a device below the switch can
> > > bypass the IOMMU and DMA directly to other devices below the switch, so
> > > all the downstream devices must be in the same IOMMU group as the switch
> > > itself.
> > Help me think through what's going on here.  IIUC, we put devices in
> > the same IOMMU group when they can interfere with each other in any
> > way (DMA, config access, etc).
> > 
> > (We said "DMA" above, but I guess this would also apply to config
> > requests, right?)
> 
> I am not sure whether devices could interfere each other through config
> space access. The IOMMU hardware only protects and isolates DMA
> accesses, so that userspace could control DMA directly. The config
> accesses will always be intercepted by VFIO. Hence, I don't see a
> problem.

I was wondering about config accesses generated by an endpoint, e.g.,
an endpoint doing config writes to a peer or the upstream bridge.

But I think that is prohibited by spec - PCIe r5.0, sec 7.3.3, says
"Propagation of Configuration Requests from Downstream to Upstream as
well as peer-to-peer are not supported" and "Configuration Requests
are initiated only by the Host Bridge, including those passed through
the SFI CAM mechanism."

Bjorn