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Subject: Re: [RFC 00/13] virtio-iommu on non-devicetree platforms
On Fri, Nov 22, 2019 at 11:49:47AM +0100, Jean-Philippe Brucker wrote:
> I'm seeking feedback on multi-platform support for virtio-iommu. At the
> moment only devicetree (DT) is supported and we don't have a pleasant
> solution for other platforms. Once we figure out the topology
> description, x86 support is trivial.
>
> Since the IOMMU manages memory accesses from other devices, the guest
> kernel needs to initialize the IOMMU before endpoints start issuing DMA.
> It's a solved problem: firmware or hypervisor describes through DT or
> ACPI tables the device dependencies, and probe of endpoints is deferred
> until the IOMMU is probed. But:
>
> (1) ACPI has one table per vendor (DMAR for Intel, IVRS for AMD and IORT
> for Arm). From my point of view IORT is easier to extend, since we
> just need to introduce a new node type. There are no dependencies to
> Arm in the Linux IORT driver, so it works well with CONFIG_X86.
>
> However, there are concerns about other OS vendors feeling obligated
> to implement this new node, so Arm proposed introducing another ACPI
> table, that can wrap any of DMAR, IVRS and IORT to extend it with
> new virtual nodes. A draft of this VIOT table specification is
> available at http://jpbrucker.net/virtio-iommu/viot/viot-v5.pdf
>
> I'm afraid this could increase fragmentation as guests would need to
> implement or modify their support for all of DMAR, IVRS and IORT. If
> we end up doing VIOT, I suggest limiting it to IORT.
>
> (2) In addition, there are some concerns about having virtio depend on
> ACPI or DT. Some hypervisors (Firecracker, QEMU microvm, kvmtool x86
> [1])
power?
> don't currently implement those methods.
>
> It was suggested to embed the topology description into the device.
> It can work, as demonstrated at the end of this RFC, with the
> following limitations:
>
> - The topology description must be read before any endpoint managed
> by the IOMMU is probed, and even before the virtio module is
> loaded. This RFC uses a PCI quirk to manually parse the virtio
> configuration. It assumes that all endpoints managed by the IOMMU
> are under this same PCI host.
>
> - I don't have a solution for the virtio-mmio transport at the
> moment, because I haven't had time to modify a host to test it. I
> think it could either use a notifier on the platform bus, or
> better, a new 'iommu' command-line argument to the virtio-mmio
> driver.
A notifier seems easier for users. What are the disadvantages of
that?
> So the current prototype doesn't work for firecracker and
> microvm, which rely on virtio-mmio.
>
> - For Arm, if the platform has an ITS, the hypervisor needs IORT or
> DT to describe it anyway. More generally, not using either ACPI or
> DT might prevent from supporting other features as well. I suspect
> the above users will have to implement a standard method sooner or
> later.
>
> - Even when reusing as much existing code as possible, guest support
> is still going to be around a few hundred lines since we can't
> rely on the normal virtio infrastructure to be loaded at that
> point. As you can see below, the diffstat for the incomplete
> topology implementation is already bigger than the exhaustive IORT
> support, even when jumping through the VIOT hoop.
>
> So it's a lightweight solution for very specific use-cases, and we
> should still support ACPI for the general case. Multi-platform
> guests such as Linux will then need to support three topology
> descriptions instead of two.
>
> In this RFC I present both solutions, but I'd rather not keep all of it.
> Please see the individual patches for details:
>
> (1) Patches 1, 3-10 add support for virtio-iommu to the Linux IORT
> driver and patches 2, 11 add the VIOT glue.
>
> (2) Patch 12 adds the built-in topology description to the virtio-iommu
> specification. Patch 13 is a partial implementation for the Linux
> virtio-iommu driver. It only supports PCI, not platform devices.
>
> You can find Linux and QEMU code on my virtio-iommu/devel branches at
> http://jpbrucker.net/git/linux and http://jpbrucker.net/git/qemu
>
>
> I split the diffstat since there are two independent features. The first
> one is for patches 1-11, and the second one for patch 13.
>
> Jean-Philippe Brucker (11):
> ACPI/IORT: Move IORT to the ACPI folder
> ACPI: Add VIOT definitions
> ACPI/IORT: Allow registration of external tables
> ACPI/IORT: Add node categories
> ACPI/IORT: Support VIOT virtio-mmio node
> ACPI/IORT: Support VIOT virtio-pci node
> ACPI/IORT: Defer probe until virtio-iommu-pci has registered a fwnode
> ACPI/IORT: Add callback to update a device's fwnode
> iommu/virtio: Create fwnode if necessary
> iommu/virtio: Update IORT fwnode
> ACPI: Add VIOT table
>
> MAINTAINERS | 9 +
> drivers/acpi/Kconfig | 7 +
> drivers/acpi/Makefile | 2 +
> drivers/acpi/arm64/Kconfig | 3 -
> drivers/acpi/arm64/Makefile | 1 -
> drivers/acpi/bus.c | 2 +
> drivers/acpi/{arm64 => }/iort.c | 317 ++++++++++++++++++++++++++------
> drivers/acpi/tables.c | 2 +-
> drivers/acpi/viot.c | 44 +++++
> drivers/iommu/Kconfig | 1 +
> drivers/iommu/virtio-iommu.c | 61 +++++-
> include/acpi/actbl2.h | 31 ++++
> include/linux/acpi_iort.h | 14 ++
> include/linux/acpi_viot.h | 20 ++
> 14 files changed, 448 insertions(+), 66 deletions(-)
> rename drivers/acpi/{arm64 => }/iort.c (86%)
> create mode 100644 drivers/acpi/viot.c
> create mode 100644 include/linux/acpi_viot.h
>
> Jean-Philippe Brucker (1):
> iommu/virtio: Add topology description to virtio-iommu config space
>
> drivers/base/platform.c | 3 +
> drivers/iommu/Kconfig | 9 +
> drivers/iommu/Makefile | 1 +
> drivers/iommu/virtio-iommu-topology.c | 410 ++++++++++++++++++++++++++
> drivers/iommu/virtio-iommu.c | 3 +
> drivers/pci/pci-driver.c | 3 +
> include/linux/virtio_iommu.h | 18 ++
> include/uapi/linux/virtio_iommu.h | 26 ++
> 8 files changed, 473 insertions(+)
> create mode 100644 drivers/iommu/virtio-iommu-topology.c
> create mode 100644 include/linux/virtio_iommu.h
>
>
> [1] firecracker: https://github.com/firecracker-microvm/firecracker
> microvm: https://github.com/qemu/qemu/blob/master/docs/microvm.rst
> kvmtool: https://git.kernel.org/pub/scm/linux/kernel/git/will/kvmtool.git/
> --
> 2.24.0
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