[RFC] ARM VM System Sepcification

[Christopher Covington]

Hi Christoffer,

On 02/26/2014 01:34 PM, Christoffer Dall wrote:
> 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.

Would you consider including simulators/emulators as well, such as QEMU in TCG
mode?

> 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.

Can you elaborate on the motivation for requiring that the kernel be stuffed
into a disk image and for requiring such a heavyweight bootloader/firmware? By
doing so you would seem to exclude those requiring an optimized boot process.

> 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.

Is VirtIO hotplug capable? Over PCI or MMIO transports or both?

> 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.

I would love to eventually see some defconfigs for this sort of thing.

> 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.

Well, the commit message for it said it mandated a GIC and architected timers.

Regards,
Christopher

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