Michael S. Tsirkin <mst at 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 at
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
