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
