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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Cc: "virtio-dev@lists.oasis-open.org"
	<virtio-dev@lists.oasis-open.org>,
	"kevin.tian@intel.com" <kevin.tian@intel.com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>,
	"tnowicki@caviumnetworks.com" <tnowicki@caviumnetworks.com>,
	"frowand.list@gmail.com" <frowand.list@gmail.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	"joro@8bytes.org" <joro@8bytes.org>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"robh+dt@kernel.org" <robh+dt@kernel.org>,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
	"bhelgaas@google.com" <bhelgaas@google.com>,
	Robin Murphy <Robin.Murphy@arm.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	"bauerman@linux.ibm.com" <bauerman@linux.ibm.com>
Subject: Re: [virtio-dev] Re: [PATCH v8 2/7] dt-bindings: virtio: Add virtio-pci-iommu node
Date: Sun, 16 Jun 2019 16:04:49 -0400
Message-ID: <20190616154841-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <c3cd5dba-123d-e808-98b1-731ac2d4b950@arm.com>

On Fri, May 31, 2019 at 12:13:47PM +0100, Jean-Philippe Brucker wrote:
> On 30/05/2019 18:45, Michael S. Tsirkin wrote:
> > On Thu, May 30, 2019 at 06:09:24PM +0100, Jean-Philippe Brucker wrote:
> >> Some systems implement virtio-iommu as a PCI endpoint. The operating
> >> system needs to discover the relationship between IOMMU and masters long
> >> before the PCI endpoint gets probed. Add a PCI child node to describe the
> >> virtio-iommu device.
> >>
> >> The virtio-pci-iommu is conceptually split between a PCI programming
> >> interface and a translation component on the parent bus. The latter
> >> doesn't have a node in the device tree. The virtio-pci-iommu node
> >> describes both, by linking the PCI endpoint to "iommus" property of DMA
> >> master nodes and to "iommu-map" properties of bus nodes.
> >>
> >> Reviewed-by: Rob Herring <robh@kernel.org>
> >> Reviewed-by: Eric Auger <eric.auger@redhat.com>
> >> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> > 
> > So this is just an example right?
> > We are not defining any new properties or anything like that.
> 
> Yes it's just an example. The properties already exist but it's good to
> describe how to put them together for this particular case, because
> there isn't a precedent describing the topology for an IOMMU that
> appears on the PCI bus.
> 
> > I think down the road for non dt platforms we want to put this
> > info in the config space of the device. I do not think ACPI
> > is the best option for this since not all systems have it.
> > But that can wait.
> 
> There is the probe order problem - PCI needs this info before starting
> to probe devices on the bus.


This isn't all that special - it's pretty common for
IOMMUs to be pci devices. The solution is to have the device on
bus 0. For example, add it with

DECLARE_PCI_FIXUP_EARLY
or
DECLARE_PCI_FIXUP_CLASS_EARLY

in e.g.
arch/x86/kernel/quirks.c
or
drivers/pci/quirks.c

You can also use the configuration access capability
if there's need to access the device before its memory is
enabled.

> Maybe we could store the info in a separate
> memory region, that is referenced on the command-line and that the guest
> can read early.
> 
> Thanks,
> Jean

The point is to avoid command line hacks. Devices should be
self describing.

-- 
MST
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Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-30 17:09 [PATCH v8 0/7] Add virtio-iommu driver Jean-Philippe Brucker
2019-05-30 17:09 ` [PATCH v8 1/7] dt-bindings: virtio-mmio: Add IOMMU description Jean-Philippe Brucker
2019-05-30 17:09 ` [PATCH v8 2/7] dt-bindings: virtio: Add virtio-pci-iommu node Jean-Philippe Brucker
2019-05-30 17:45   ` Michael S. Tsirkin
2019-05-31 11:13     ` [virtio-dev] " Jean-Philippe Brucker
2019-06-16 20:04       ` Michael S. Tsirkin [this message]
2019-05-30 17:09 ` [PATCH v8 3/7] of: Allow the iommu-map property to omit untranslated devices Jean-Philippe Brucker
2019-05-30 17:09 ` [PATCH v8 4/7] PCI: OF: Initialize dev->fwnode appropriately Jean-Philippe Brucker
2019-05-30 17:09 ` [PATCH v8 5/7] iommu: Add virtio-iommu driver Jean-Philippe Brucker
2019-06-14  7:33   ` Auger Eric
2019-05-30 17:09 ` [PATCH v8 6/7] iommu/virtio: Add probe request Jean-Philippe Brucker
2019-05-30 17:09 ` [PATCH v8 7/7] iommu/virtio: Add event queue Jean-Philippe Brucker
2019-06-13 15:54 ` [PATCH v8 0/7] Add virtio-iommu driver Auger Eric

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