RE: [PATCH 5/5 v11] iommu/fsl: Freescale PAMU driver and iommu implementation.

[Yoder Stuart-B08248 <B08248@freescale.com>]

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> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@redhat.com]
> Sent: Thursday, April 04, 2013 10:22 AM
> To: Sethi Varun-B16395
> Cc: Joerg Roedel; Yoder Stuart-B08248; Wood Scott-B07421; iommu@lists.linux-foundation.org; linuxppc-
> dev@lists.ozlabs.org; linux-kernel@vger.kernel.org; galak@kernel.crashing.org; benh@kernel.crashing.org
> Subject: Re: [PATCH 5/5 v11] iommu/fsl: Freescale PAMU driver and iommu implementation.
> 
> On Thu, 2013-04-04 at 13:00 +0000, Sethi Varun-B16395 wrote:
> >
> > > -----Original Message-----
> > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > Sent: Wednesday, April 03, 2013 11:32 PM
> > > To: Joerg Roedel
> > > Cc: Sethi Varun-B16395; Yoder Stuart-B08248; Wood Scott-B07421;
> > > iommu@lists.linux-foundation.org; linuxppc-dev@lists.ozlabs.org; linux-
> > > kernel@vger.kernel.org; galak@kernel.crashing.org;
> > > benh@kernel.crashing.org
> > > Subject: Re: [PATCH 5/5 v11] iommu/fsl: Freescale PAMU driver and iommu
> > > implementation.
> > >
> > > On Tue, 2013-04-02 at 18:18 +0200, Joerg Roedel wrote:
> > > > Cc'ing Alex Williamson
> > > >
> > > > Alex, can you please review the iommu-group part of this patch?
> > >
> > > Sure, it looks pretty reasonable.  AIUI, all PCI devices are below some
> > > kind of host bridge that is either new and supports partitioning or old
> > > and doesn't.  I don't know if that's a visibility or isolation
> > > requirement, perhaps PCI ACS-ish.  In the new host bridge case, each
> > > device gets a group.  This seems not to have any quirks for multifunction
> > > devices though.  On AMD and Intel IOMMUs we test multifunction device ACS
> > > support to determine whether all the functions should be in the same
> > > group.  Is there any reason to trust multifunction devices on PAMU?
> > >
> > [Sethi Varun-B16395] In the case where we can partition endpoints we
> > can distinguish transactions based on the bus,device,function number
> > combination. This support is available in the PCIe controller (host
> > bridge).
> 
> So can x86 IOMMUs, that's the visibility aspect of IOMMU groups.
> Visibility alone doesn't necessarily imply that a device is isolated
> though.  A multifunction PCI device that doesn't expose ACS support may
> not isolate functions from each other.  For example a peer-to-peer DMA
> between functions may not be translated by the upstream IOMMU.  IOMMU
> groups should encompass both visibility and isolation.
> 
> > > I also find it curious what happens to the iommu group of the host
> > > bridge.  In the partitionable case the host bridge group is removed, in
> > > the non-partitionable case the host bridge group becomes the group for
> > > the children, removing the host bridge.  It's unique to PAMU so far that
> > > these host bridges are even in an iommu group (x86 only adds pci
> > > devices), but I don't see it as necessarily wrong leaving it in either
> > > scenario.  Does it solve some problem to remove them from the groups?
> > > Thanks,
> > [Sethi Varun-B16395] The PCIe controller isn't a partitionable entity,
> > it would always be owned by the host.
> 
> Ownership of a device shouldn't play into the group context.  An IOMMU
> group should be defined by it's visibility and isolation from other
> devices.  Whether the PCIe controller is allowed to be handed to
> userspace is a question for VFIO.  Thanks,

Right now the add_device() callback gets called for all platform
devices (including PCI controller) and PCI devices.  PCI controllers
are a kind of special case in that their device tree node has a
property indicating that it is DMA capable...but in fact the
PCI controller itself does not DMA, but it's the PCI endpoints
under it.

So, as you noted the bridge/controller shouldn't be in an IOMMU
group and so since the platform 'add device' code didn't special
case PCI controllers, this patch removes their group if it's there.

Stuart
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