Using KVM with Qemu on ARM

This is part two of my blog post about Kernel-Virtual Machine (KVM) on a 32-Bit ARM architecture. The post is meant as a starting point for those who want to play with KVM and provide a useful collection of Qemu commands for virtualization.

Virtualization host setup

The Kernel configuration I used for my platforms Host kernel can be found here. Since I run my experiments on a Toradex Colibri iMX7D module, I started with the v4.1 configuration of the BSP kernel and updated that to v4.8 plus enabled KVM as well as KSM (Kernel same-page merging).

As root file system I use a slightly modified version of the Ångström distributions “development-image”, version 2015.12 (built from scratch with OpenEmbedded). Any recent ARM root file system should do it. I let Qemu v2.6.0 preinstall (by just adding “qemu” to the image and specifying ANGSTROM_QEMU_VERSION = “2.6.0” in conf/distro/angstrom-v2015.12.conf).

Virtualization guest setup

For the virtualization guest setup I was looking for something minimalistic. I uploaded the compiled binary of the Kernel (as tared zImage) and initramfs (as cpio.gz).

I built a custom kernel directly using v4.7 sources and a modified/stripped down version of the vexpress_defconfig (virt_guest_defconfig). I found it useful to look into Qemu’s “virt” machine setup code (hw/arm/virt.c) to understand what peripherals are actually emulated (and hence what drivers are actually required). Continue reading “Using KVM with Qemu on ARM”

U-Boot/Linux and HYP mode on ARMv7

The newer ARMv7 Cortex-A class cores such Cortex-A7, A15 and A17 come with a virtualization extensions which allow to use KVM (kernel virtual machine). The NXP i.MX 7Dual SoC which I worked with lately includes the ARM Cortex-A7 CPU. I went ahead and tried to bring up KVM on i.MX 7. I was not really familiar with the ARMv7 virtualization architecture, so I had to read up on some concepts. This post summarizes what I learned and gives a big picture of software support.

The Hypervisor mode

To provide hardware support for full CPU virtualization an additional privilege level is required. User-space (PL0) uses the SVC (Supervisor) instruction to switch to kernel-space (PL1, SVC mode). A similar separation between Kernel and hypervisor is required. The ARMv7 architecture with virtualization extension calls this privilege level PL2 or HYP mode.

Linux with KVM for ARM uses this mode to provide CPU virtualization. The CPU needs to be in HYP mode when Linux is booting so KVM can make use of the extension. How KVM uses the HYP mode in detail is explained in this excellent LWN article. After building a kernel with KVM support, I encountered this problem first: By default, the system did boot in SVC mode.

...
Brought up 2 CPUs
CPU: All CPU(s) started in SVC mode.
...
kvm [1]: HYP mode not available
...

Secure and Non-Secure world

ARMv7 privilege levels
ARMv7 privilege levels

To understand how to switch into Hypervisor mode, one needs to understand the whole privilege level architecture first. Notable here is that on ARMv7 CPU’s the HYP mode is only available in non-secure mode, by design. Any hypervisor needs to operate in non-secure mode, there is no virtualization extension in secure mode. Continue reading “U-Boot/Linux and HYP mode on ARMv7”

Using the perf utility on ARM

Given two systems, both with a Cortex-A5 CPU, one clocked at 396MHz without L2 cache and one clocked at 500 MHz with 512kB L2 cache. How big is the impact of the L2 cache? Since the clock frequency is different, a simple CPU time comparison of a given program does not answer the question… I tried to answer this question using perf. perf is often used to profile software, but in this case it also proved to be useful to compare two different hardware implementations.

Most CPU’s nowadays have internal counters which count various events (e.g. executed instructions, cache misses, executed branches and branch misses etc…). Other hardware, e.g. cache controllers, might expose performance counters too, but this article focuses on the hardware counters exposed by the CPU. Continue reading “Using the perf utility on ARM”

Linux earlyprintk/earlycon support on ARM

The serial console is a very helpful debugging tool for kernel development. However, when a crash occurs early in the boot process, one is left in the dark. There are two early console implementations available with different merits. This post will throw some lights on them.

earlyprintk

The kernel supports earlyprintk since… probably ever. At least 2.6.12, where the new age (git) started. After enabling “Kernel low-level debugging”, “Early printk” under “Kernel hacking” and selecting an appropriate low-level debugging port, you are ready to get early serial console output. Continue reading “Linux earlyprintk/earlycon support on ARM”

Debug IPv6 issues on OpenWrt

My new provider IPv6 connectivity, however after connecting my OpenWrt based WNDR4300 (using Chaos Calmer 15.05-rc3) things unfortunately did not “just work”… This post summarize some hints how to debug IPv6 on OpenWrt.

First, make sure that the upstream (WAN) interface is configured according to the documentation in the OpenWrt Wiki. Using LuCI, you have to create a new interface (WAN6) which should c

If your ISP provides IPv6 support, it should “just work” at this point. However, what to do if not? Continue reading “Debug IPv6 issues on OpenWrt”

Cross compile Linux for ARM using LLVM/clang on Arch Linux

The LLVMLinux team made quite some progress in enabling LLVM/clang to build the Linux kernel. There has also been a talk at the Collaboration Summit 2015 in Santa Rosa (slides). So I thought, give it a try and compile a kernel for my ARM based board of choice. The upstream kernel.org Linux kernel is not yet ready to be built using LLVM/clang, some patches are still required. Therefor I used the kernel from the LLVMLinux git server.

The nice thing about LLVM/clang is that it supports multiple targets. The binaries provided by the Arch repositories support several ARM targets:

$ pacman -S clang
...
$ llc --version
LLVM (http://llvm.org/):
 LLVM version 3.5.1
 Optimized build.
 Built Jan 14 2015 (03:18:15).
 Default target: x86_64-unknown-linux-gnu
 Host CPU: core-avx-i

 Registered Targets:
 aarch64 - AArch64 (little endian)
 aarch64_be - AArch64 (big endian)
 arm - ARM
 arm64 - AArch64 (little endian)
 arm64_be - AArch64 (big endian)
 armeb - ARM (big endian)
...

Continue reading “Cross compile Linux for ARM using LLVM/clang on Arch Linux”