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memory_and_devices.md

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DomU Memory, MMIO, and Interrupts

This document details where normal memory and MMIO regions of devices are located in the address space of a DomU. It also details interrupts available, if any. The most relevant header file is xen/include/public/arch-arm.h in xen.git.

Normal Memory

The first bank is up to 3GB in size and starts at 0x40000000. The second bank is up to 1016GB in size and starts at 0x0200000000.

First bank (up to 3GB) 0x40000000 - 0x100000000
Second bank (up to 1016GB) 0x0200000000 - 0x10000000000

Devices

GIC

Xen provides to DomUs an interrupt controller compatible with the Arm Generic Interrupt Controller GIC. The GIC version exposed to the DomU matches the version of the physical GIC on the platform: if the hardware is GICv2 a domU will get a GICv2; if the hardware is GICv3 a domU will get a GICv3.

GICv2

GICD MMIO 0x03001000 - 0x03002000 GICC MMIO 0x03002000 - 0x03004000

GICv3

GICD MMIO 0x03001000 - 0x03011000 GICR MMIO 0x03020000 - 0x04020000

Arch Timer

A timer compatible with Arm Arch Timer is exposed to all DomUs. Both the virtual timer and the physical timer interfaces are provided. Only the CP15 and SYSREG interfaces are made avialble to DomUs, no MMIO regions.

virt_timer PPI 27 phys_timer PPI 30

PL011

Xen can provide a UART PL011 to DomUs, including its MMIO region and interrupt. It is not enabled by default.

MMIO 0x22000000 - 0x22001000 SPI 32

PSCI

The Power State Coordination Interface PSCI is a standard interface for power management. Rich operating systems, like Linux and Windows, use PSCI for CPU and overall system power management.

Xen exposes a virtual PSCI interface to DomUs for them to bring up secondary virtual CPUs. PSCI supports multiple transports, Xen chooses the hvc instruction as transport for DomUs.

Device Tree

The information listed above is also advertised via device tree. At boot time, a device tree binary (FDT binary blob) is mapped into domU memory. The address in guest memory of the binary is passed to the domU kernel via the x21 register.

The following is a sample device tree for a DomU, converted to device tree source (DTS) form:

/dts-v1/;

/ {
        #address-cells = <0x2>;
        model = "XENVM-4.13";
        #size-cells = <0x2>;
        interrupt-parent = <0xfde8>;
        compatible = "xen,xenvm-4.13", "xen,xenvm";

        interrupt-controller@3001000 {
                #address-cells = <0x0>;
                compatible = "arm,cortex-a15-gic", "arm,cortex-a9-gic";
                #interrupt-cells = <0x3>;
                reg = <0x0 0x3001000 0x0 0x1000 0x0 0x3002000 0x0 0x2000>;
                phandle = <0xfde8>;
                linux,phandle = <0xfde8>;
                interrupt-controller;
        };

        memory@40000000 {
                device_type = "memory";
                reg = <0x0 0x40000000 0x0 0x64000000>;
        };

        psci {
                method = "hvc";
                compatible = "arm,psci-1.0", "arm,psci-0.2", "arm,psci";
                cpu_on = <0x2>;
                cpu_off = <0x1>;
        };

        timer {
                interrupts = <0x1 0xd 0xf08 0x1 0xe 0xf08 0x1 0xb 0xf08>;
                interrupt-parent = <0xfde8>;
                compatible = "arm,armv8-timer";
        };

        chosen {
                linux,initrd-end = <0x0 0x57774000>;
                bootargs = "console=hvc0 root=/dev/ram0";
                linux,initrd-start = <0x0 0x48000000>;
        };

        cpus {
                #address-cells = <0x1>;
                #size-cells = <0x0>;

                cpu@0 {
                        device_type = "cpu";
                        compatible = "arm,armv8";
                        reg = <0x0>;
                        enable-method = "psci";
                };
        };
};

The number of CPUs and the amount of memory vary depending on the virtual machine configuration. The version of the GIC also varies and depends on the underlying physical hardware as well as the virtual machine configuration. The chosen node contains the location of the Linux initrd in guest memory and the DomU kernel command line arguments as specified in the virtual machine configuration file.