64-bit ARM usermode emulation in QEMU 2.0.0

The QEMU Project released version 2.0.0 of QEMU last week; this seems like a good time to summarise our progress with ARMv8 QEMU work.

One of the major new ARM related features in this release is support for emulating AArch64 processes in QEMU’s “linux-user” mode; in Linaro we’ve been working on this over the last few months (building on a great foundation established by SUSE) and we just managed to squeeze support for the last few instructions into 2.0.0.

“linux-user” mode is where we run a single Linux guest binary, and QEMU converts the system calls the guest makes into system calls to the host Linux kernel. Typically you’d use this to run an AArch64 binary on a more conveniently available host, usually x86_64, by setting up a cross-architecture chroot and putting QEMU in it. We’ve implemented support for all the mandatory A64 instructions, including floating point and Advanced SIMD, but not the optional instructions in the crypto and CRC extensions.

As well as adding an entirely new instruction set for 64 bit support, the ARMv8 architecture included a few new instructions for the 32 bit A32 and T32 instruction sets. QEMU also now implements all the mandatory new instructions, though this will for the moment probably mostly be of use only to people running compiler test suites.

Two other uses for QEMU involve running it on AArch64 hardware. Firstly, you can use it to emulate other CPU architectures on AArch64 hosts, for instance running an x86 kernel in an emulated machine. This was contributed by Huawei last year, and has been supported since the previous release of QEMU (1.7).

You can also use QEMU as the userspace device emulation part of a virtual machine which uses KVM and the hardware’s virtualization extensions to provide fast AArch64-on-AArch64 VMs. This too has been supported since 1.7, though some features are not yet implemented (for instance, VM migration and debugging a guest VM are both not currently supported).

The final use for QEMU I want to talk about is the only one which isn’t in the 2.0.0 release, but many people have been waiting for it so here’s a status update. AArch64 system emulation is where you emulate a complete system and boot a full system including an AArch64 Linux kernel and user space, typically running on an x86 host. We’re working on this right now, and in fact as soon as QEMU’s git repository reopened for development after the 2.0.0 release we landed a large set of patches which implement all the necessary CPU emulation support. The only remaining missing piece in upstream QEMU master to be able to boot a kernel is to add support for running the “virt” board model with a Cortex-A57 and a GICv2 with an appropriate register layout. This last bit of work should be done shortly.

If you want to try out QEMU 2.0.0 you can build it yourself from the upstream released tarballs. If you’re an Ubuntu user then you’re in luck, because these changes are also in the QEMU shipped in the newly released Ubuntu 14.04 LTS.