Re: KVM on ARM Cortex A53 in 32-bit Mode

[Christoffer Dall <christoffer.dall@linaro.org>]

Hi Yasutaka,

On Thu, Mar 02, 2017 at 09:57:50AM -0500, Yasutaka Tanaka wrote:
> Hello KVM/ARM community,
> 
> I am now trying to execute KVM on Raspberry Pi 3 Model B (RPI3).
> Does anyone know whether KVM/ARM can run with 32-bit mode on ARM Cortex A53?

Theoretically, it should.

> 
> The CPU of RPI3 is ARM Cortex A53 which can be executed in both 32-bit and
> 64-bit mode. I've managed to display /dev/kvm on 64-bit mode execution
> (i.e. with a 64-bit linux kernel) on RPI3, but I failed to do so in 32-bit
> mode.
> 
> In 32-bit mode, an KVM error messages says CPU not supported. I guess the
> reason for the error message is KVM_ARM_TARGET_CORTEX_A53 is only returned
> in /arch/arm64/kvm/guest.c not under/arch/arm/kvm/guest.c.
> 
> So I'm wondering whether it's possible or not. 

It should be.  You may just have to add the minimal missing definitions
for a 32-bit A53 CPU support.

> What I'd like to do is to
> compare the execution speed of KVM between 32-bit and 64-bit mode. The
> second plan is to compare between KVMs of 32-bit RPI2 and 64-bit RPI3 (I
> have both), but since RPI2 spec is a little low and I'm worried that some
> numeric comparisons become unfair.

I would be careful here, since the RPI3 doesn't have the GIC and you'll
end up going to user space a lot for interrupt handling.  Especially
look out for SMP workloads in the VM which may generate a lot of IPIs
which are really going to suffer from having to go to userspace on the
host.

> 
> (I was also notified by an KVM/ARM expert that since RPI doesn't have a GIC
> and I need to apply some patches)

Yes, you need the "Support userspace irqchip with arch timers" series
and corresponding QEMU patches:

http://www.spinics.net/lists/kvm/msg144685.html

Good luck.
-Christoffer