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

From: Bjorn Helgaas <helgaas@kernel.org>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Joerg Roedel <joro@8bytes.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Christoph Hellwig <hch@infradead.org>,
	Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.com>, Will Deacon <will@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Dan Williams <dan.j.williams@intel.com>,
	rafael@kernel.org, Diana Craciun <diana.craciun@oss.nxp.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Eric Auger <eric.auger@redhat.com>, Liu Yi L <yi.l.liu@intel.com>,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Stuart Yoder <stuyoder@gmail.com>,
	Laurentiu Tudor <laurentiu.tudor@nxp.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Li Yang <leoyang.li@nxp.com>, Dmitry Osipenko <digetx@gmail.com>,
	iommu@lists.linux-foundation.org, linux-pci@vger.kernel.org,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 09/14] PCI: portdrv: Suppress kernel DMA ownership auto-claiming
Date: Thu, 6 Jan 2022 12:32:24 -0600	[thread overview]
Message-ID: <20220106183224.GA298861@bhelgaas> (raw)
In-Reply-To: <039bbcf3-ccc6-f3b0-172e-9caa0866bb9e@linux.intel.com>

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