[Xen-devel] [RFC] ARM VM System Sepcification

[Christoffer Dall <christoffer.dall@xxxxxxxxxx>]

ARM VM System Specification
===========================

Goal
----
The goal of this spec is to allow suitably-built OS images to run on
all ARM virtualization solutions, such as KVM or Xen.

Recommendations in this spec are valid for aarch32 and aarch64 alike, and
they aim to be hypervisor agnostic.

Note that simply adhering to the SBSA [2] is not a valid approach,
for example because the SBSA mandates EL2, which will not be available
for VMs.  Further, the SBSA mandates peripherals like the pl011, which
may be controversial for some ARM VM implementations to support.
This spec also covers the aarch32 execution mode, not covered in the
SBSA.


Image format
------------
The image format, as presented to the VM, needs to be well-defined in
order for prepared disk images to be bootable across various
virtualization implementations.

The raw disk format as presented to the VM must be partitioned with a
GUID Partition Table (GPT).  The bootable software must be placed in the
EFI System Partition (ESP), using the UEFI removable media path, and
must be an EFI application complying to the UEFI Specification 2.4
Revision A [6].

The ESP partition's GPT entry's partition type GUID must be
C12A7328-F81F-11D2-BA4B-00A0C93EC93B and the file system must be
formatted as FAT32/vfat as per Section 12.3.1.1 in [6].

The removable media path is \EFI\BOOT\BOOTARM.EFI for the aarch32
execution state and is \EFI\BOOT\BOOTAA64.EFI for the aarch64 execution
state.

This ensures that tools for both Xen and KVM can load a binary UEFI
firmware which can read and boot the EFI application in the disk image.

A typical scenario will be GRUB2 packaged as an EFI application, which
mounts the system boot partition and boots Linux.


Virtual Firmware
----------------
The VM system must be able to boot the EFI application in the ESP.  It
is recommended that this is achieved by loading a UEFI binary as the
first software executed by the VM, which then executes the EFI
application.  The UEFI implementation should be compliant with UEFI
Specification 2.4 Revision A [6] or later.

This document strongly recommends that the VM implementation supports
persistent environment storage for virtual firmware implementation in
order to ensure probable use cases such as adding additional disk images
to a VM or running installers to perform upgrades.

The binary UEFI firmware implementation should not be distributed as
part of the VM image, but is specific to the VM implementation.


Hardware Description
--------------------
The Linux kernel's proper entry point always takes a pointer to an FDT,
regardless of the boot mechanism, firmware, and hardware description
method.  Even on real hardware which only supports ACPI and UEFI, the kernel
entry point will still receive a pointer to a simple FDT, generated by
the Linux kernel UEFI stub, containing a pointer to the UEFI system
table.  The kernel can then discover ACPI from the system tables.  The
presence of ACPI vs. FDT is therefore always itself discoverable,
through the FDT.

Therefore, the VM implementation must provide through its UEFI
implementation, either:

  a complete FDT which describes the entire VM system and will boot
  mainline kernels driven by device tree alone, or

  no FDT.  In this case, the VM implementation must provide ACPI, and
  the OS must be able to locate the ACPI root pointer through the UEFI
  system table.

For more information about the arm and arm64 boot conventions, see
Documentation/arm/Booting and Documentation/arm64/booting.txt in the
Linux kernel source tree.

For more information about UEFI and ACPI booting, see [4] and [5].


VM Platform
-----------
The specification does not mandate any specific memory map.  The guest
OS must be able to enumerate all processing elements, devices, and
memory through HW description data (FDT, ACPI) or a bus-specific
mechanism such as PCI.

The virtual platform must support at least one of the following ARM
execution states:
  (1) aarch32 virtual CPUs on aarch32 physical CPUs
  (2) aarch32 virtual CPUs on aarch64 physical CPUs
  (3) aarch64 virtual CPUs on aarch64 physical CPUs

It is recommended to support both (2) and (3) on aarch64 capable
physical systems.

The virtual hardware platform must provide a number of mandatory
peripherals:

  Serial console:  The platform should provide a console,
  based on an emulated pl011, a virtio-console, or a Xen PV console.

  An ARM Generic Interrupt Controller v2 (GICv2) [3] or newer.  GICv2
  limits the the number of virtual CPUs to 8 cores, newer GIC versions
  removes this limitation.

  The ARM virtual timer and counter should be available to the VM as
  per the ARM Generic Timers specification in the ARM ARM [1].

  A hotpluggable bus to support hotplug of at least block and network
  devices.  Suitable buses include a virtual PCIe bus and the Xen PV
  bus.


We make the following recommendations for the guest OS kernel:

  The guest OS must include support for GICv2 and any available newer
  version of the GIC architecture to maintain compatibility with older
  VM implementations.

  It is strongly recommended to include support for all available
  (block, network, console, balloon) virtio-pci, virtio-mmio, and Xen PV
  drivers in the guest OS kernel or initial ramdisk.


Other common peripherals for block devices, networking, and more can
(and typically will) be provided, but OS software written and compiled
to run on ARM VMs cannot make any assumptions about which variations
of these should exist or which implementation they use (e.g. VirtIO or
Xen PV).  See "Hardware Description" above.

Note that this platform specification is separate from the Linux kernel
concept of mach-virt, which merely specifies a machine model driven
purely from device tree, but does not mandate any peripherals or have any
mention of ACPI.


References
----------
[1]: The ARM Architecture Reference Manual, ARMv8, Issue A.b
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0487a.b/index.html

[2]: ARM Server Base System Architecture
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0029/index.html

[3]: The ARM Generic Interrupt Controller Architecture Specifications v2.0
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0487a.b/index.html

[4]: http://www.secretlab.ca/archives/27

[5]: 
https://git.linaro.org/people/leif.lindholm/linux.git/blob/refs/heads/uefi-for-upstream:/Documentation/arm/uefi.txt

[6]: UEFI Specification 2.4 Revision A
http://www.uefi.org/sites/default/files/resources/2_4_Errata_A.pdf

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