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From: Thiago Jung Bauermann <bauerman@linux.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
	mark.rutland@arm.com, virtio-dev@lists.oasis-open.org,
	kevin.tian@intel.com, tnowicki@caviumnetworks.com,
	devicetree@vger.kernel.org, marc.zyngier@arm.com,
	linux-pci@vger.kernel.org, will.deacon@arm.com,
	robin.murphy@arm.com, virtualization@lists.linux-foundation.org,
	iommu@lists.linux-foundation.org, robh+dt@kernel.org,
	kvmarm@lists.cs.columbia.edu, bhelgaas@google.com,
	frowand.list@gmail.com, jasowang@redhat.com,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v7 0/7] Add virtio-iommu driver
Date: Thu, 21 Feb 2019 18:57:17 -0300
Message-ID: <87a7ioby9u.fsf@morokweng.localdomain> (raw)
In-Reply-To: <20190129135345-mutt-send-email-mst@kernel.org>


Michael S. Tsirkin <mst@redhat.com> writes:

> On Mon, Jan 21, 2019 at 11:29:05AM +0000, Jean-Philippe Brucker wrote:
>> Hi,
>>
>> On 18/01/2019 15:51, Michael S. Tsirkin wrote:
>> >
>> > On Tue, Jan 15, 2019 at 12:19:52PM +0000, Jean-Philippe Brucker wrote:
>> >> Implement the virtio-iommu driver, following specification v0.9 [1].
>> >>
>> >> This is a simple rebase onto Linux v5.0-rc2. We now use the
>> >> dev_iommu_fwspec_get() helper introduced in v5.0 instead of accessing
>> >> dev->iommu_fwspec, but there aren't any functional change from v6 [2].
>> >>
>> >> Our current goal for virtio-iommu is to get a paravirtual IOMMU working
>> >> on Arm, and enable device assignment to guest userspace. In this
>> >> use-case the mappings are static, and don't require optimal performance,
>> >> so this series tries to keep things simple. However there is plenty more
>> >> to do for features and optimizations, and having this base in v5.1 would
>> >> be good. Given that most of the changes are to drivers/iommu, I believe
>> >> the driver and future changes should go via the IOMMU tree.
>> >>
>> >> You can find Linux driver and kvmtool device on v0.9.2 branches [3],
>> >> module and x86 support on virtio-iommu/devel. Also tested with Eric's
>> >> QEMU device [4]. Please note that the series depends on Robin's
>> >> probe-deferral fix [5], which will hopefully land in v5.0.
>> >>
>> >> [1] Virtio-iommu specification v0.9, sources and pdf
>> >>     git://linux-arm.org/virtio-iommu.git virtio-iommu/v0.9
>> >>     http://jpbrucker.net/virtio-iommu/spec/v0.9/virtio-iommu-v0.9.pdf
>> >>
>> >> [2] [PATCH v6 0/7] Add virtio-iommu driver
>> >>     https://lists.linuxfoundation.org/pipermail/iommu/2018-December/032127.html
>> >>
>> >> [3] git://linux-arm.org/linux-jpb.git virtio-iommu/v0.9.2
>> >>     git://linux-arm.org/kvmtool-jpb.git virtio-iommu/v0.9.2
>> >>
>> >> [4] [RFC v9 00/17] VIRTIO-IOMMU device
>> >>     https://www.mail-archive.com/qemu-devel@nongnu.org/msg575578.html
>> >>
>> >> [5] [PATCH] iommu/of: Fix probe-deferral
>> >>     https://www.spinics.net/lists/arm-kernel/msg698371.html
>> >
>> > Thanks for the work!
>> > So really my only issue with this is that there's no
>> > way for the IOMMU to describe the devices that it
>> > covers.
>> >
>> > As a result that is then done in a platform-specific way.
>> >
>> > And this means that for example it does not solve the problem that e.g.
>> > some power people have in that their platform simply does not have a way
>> > to specify which devices are covered by the IOMMU.
>>
>> Isn't power using device tree? I haven't looked much at power because I
>> was told a while ago that they already paravirtualize their IOMMU and
>> don't need virtio-iommu, except perhaps for some legacy platforms. Or
>> something along those lines. But I would certainly be interested in
>> enabling the IOMMU for more architectures.
>
> I have CC'd the relevant ppc developers, let's see what do they think.

I'm far from being an expert, but what could be very useful for us is to
have a way for the guest to request IOMMU bypass for a device.

From what I understand, the pSeries platform used by POWER guests always
puts devices behind an IOMMU, so at least for current systems a
description of which devices are covered by the IOMMU would always say
"all of them".

>> As for the enumeration problem, I still don't think we can get much
>> better than DT and ACPI as solutions (and IMO they are necessary to make
>> this device portable). But I believe that getting DT and ACPI support is
>> just a one-off inconvenience. That is, once the required bindings are
>> accepted, any future extension can then be done at the virtio level with
>> feature bits and probe requests, without having to update ACPI or DT.

There is a device tree binding that can specify devices connected to a
given IOMMU in Documentation/devicetree/bindings/iommu/iommu.txt.
I don't believe POWER machines use it though.

>> Thanks,
>> Jean
>>
>> > Solving that problem would make me much more excited about
>> > this device.
>> >
>> > On the other hand I can see that while there have been some
>> > developments most of the code has been stable for quite a while now.
>> >
>> > So what I am trying to do right about now, is making a small module that
>> > loads early and pokes at the IOMMU sufficiently to get the data about
>> > which devices use the IOMMU out of it using standard virtio config
>> > space.  IIUC it's claimed to be impossible without messy changes to the
>> > boot sequence.
>> >
>> > If I succeed at least on some platforms I'll ask that this design is
>> > worked into this device, minimizing info that goes through DT/ACPI.  If
>> > I see I can't make it in time to meet the next merge window, I plan
>> > merging the existing patches using DT (barring surprises).
>> >
>> > As I only have a very small amount of time to spend on this attempt, If
>> > someone else wants to try doing that in parallel, that would be great!

--
Thiago Jung Bauermann
IBM Linux Technology Center


  reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-15 12:19 Jean-Philippe Brucker
2019-01-15 12:19 ` [PATCH v7 1/7] dt-bindings: virtio-mmio: Add IOMMU description Jean-Philippe Brucker
2019-01-15 12:19 ` [PATCH v7 2/7] dt-bindings: virtio: Add virtio-pci-iommu node Jean-Philippe Brucker
2019-01-15 12:19 ` [PATCH v7 3/7] of: Allow the iommu-map property to omit untranslated devices Jean-Philippe Brucker
2019-01-15 12:19 ` [PATCH v7 4/7] PCI: OF: Initialize dev->fwnode appropriately Jean-Philippe Brucker
2019-01-15 12:19 ` [PATCH v7 5/7] iommu: Add virtio-iommu driver Jean-Philippe Brucker
2019-01-15 12:19 ` [PATCH v7 6/7] iommu/virtio: Add probe request Jean-Philippe Brucker
2019-01-15 12:19 ` [PATCH v7 7/7] iommu/virtio: Add event queue Jean-Philippe Brucker
2019-01-18 15:51 ` [PATCH v7 0/7] Add virtio-iommu driver Michael S. Tsirkin
2019-01-21 11:29   ` Jean-Philippe Brucker
2019-01-29 18:54     ` Michael S. Tsirkin
2019-02-21 21:57       ` Thiago Jung Bauermann [this message]
2019-01-23  8:34 ` Joerg Roedel
2019-01-24 16:03   ` Jean-Philippe Brucker
2019-02-21 22:18     ` Thiago Jung Bauermann
2019-02-22 12:18       ` Jean-Philippe Brucker
2019-02-25 13:20 ` Tomasz Nowicki
2019-05-12 16:31 ` Michael S. Tsirkin
2019-05-27  9:26   ` Joerg Roedel
2019-05-27 15:15     ` Michael S. Tsirkin
2019-05-28  9:18       ` Jean-Philippe Brucker
2019-05-12 17:15 ` Michael S. Tsirkin

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