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:

quoted

-----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:

quoted

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.

quoted

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,

Alex