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Stefano Stabellini
ded7303f16
Add a warning: use passthrough with care. Add a pointer to the gic device tree bindings. Add an explanation on how to calculate irq numbers from device tree. Add a brief explanation of the reg property and a pointer to the xl docs for a description of the iomem property. Add a note that in the example we are using different memory addresses for guests and host. Signed-off-by: Stefano Stabellini <sstabellini@kernel.org> Acked-by: Julien Grall <julien.grall@arm.com>
86 lines
3.4 KiB
Plaintext
86 lines
3.4 KiB
Plaintext
Passthrough a device described in the Device Tree to a guest
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============================================================
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The example will use the secondary network card for the midway server.
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1) Mark the device to let Xen know the device will be used for passthrough.
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This is done in the device tree node describing the device by adding the
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property "xen,passthrough". The command to do it in U-Boot is:
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fdt set /soc/ethernet@fff51000 xen,passthrough
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2) Create a partial device tree describing the device. The IRQ are mapped
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1:1 to the guest (i.e VIRQ == IRQ). For MMIO, you will have to find a hole
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in the guest memory layout (see xen/include/public/arch-arm.h, note that
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the layout is not stable and can change between versions of Xen). Please
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be aware that passing a partial device tree to a VM is a powerful tool,
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use it with care. In production, only allow assignment of devices which
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have been previously tested and known to work correctly when given to
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guests.
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/dts-v1/;
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/ {
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/* #*cells are here to keep DTC happy */
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#address-cells = <2>;
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#size-cells = <2>;
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aliases {
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net = &mac0;
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};
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passthrough {
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compatible = "simple-bus";
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ranges;
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#address-cells = <2>;
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#size-cells = <2>;
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mac0: ethernet@10000000 {
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compatible = "calxeda,hb-xgmac";
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reg = <0 0x10000000 0 0x1000>;
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interrupts = <0 80 4 0 81 4 0 82 4>;
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};
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};
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};
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Note:
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* The interrupt-parent property will be added by the toolstack in the
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root node;
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* The following properties are mandatory with the /passthrough node:
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- compatible: It should always contain "simple-bus"
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- ranges
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- #address-cells
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- #size-cells
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* See http://www.devicetree.org/Device_Tree_Usage for more
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information about device tree.
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* In this example, the device MMIO region is placed at a different
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address (0x10000000) compared to the host address (0xfff51000)
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3) Compile the partial guest device with dtc (Device Tree Compiler).
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For our purpose, the compiled file will be called guest-midway.dtb and
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placed in /root in DOM0.
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3) Add the following options in the guest configuration file:
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device_tree = "/root/guest-midway.dtb"
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dtdev = [ "/soc/ethernet@fff51000" ]
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irqs = [ 112, 113, 114 ]
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iomem = [ "0xfff51,1@0x10000" ]
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Please refer to your platform docs for the MMIO ranges and interrupts.
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They can also be calculated from the original device tree (not
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recommended). You can read about the "interrupts" property format in the
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device tree bindings of the interrupt controller of your platform. For
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example, in the case of GICv2 see [arm,gic.txt]; in the case of GICv3
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see [arm,gic-v3.txt] in the Linux repository. For both GICv2 and GICv3
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the "interrupts" property format is the same: the first cell is the
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interrupt type, and the second cell is the interrupt number. Given that
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SPI numbers start from 32, in this example 80 + 32 = 112.
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See man [xl.cfg] for the iomem format. The reg property is just a pair
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of address, then size numbers, each of them can occupy 1 or 2 cells.
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[arm,gic.txt]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/interrupt-controller/arm,gic.txt
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[arm,gic-v3.txt]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.txt
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[xl.cfg]: https://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html
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