Re: Qemu aarch64 emulation question

[Peter Maydell]

On Sun, 5 Sept 2021 at 20:53, David Faller <danny.faller@gmail.com> wrote:
> I have a little question about aarch64 virtualization on x86_host.
>
> At home I build up a virtual machine running aarch64 debian.
> My system setup is an AMD Ryzen 5900x.
> On this System aarch64 emulation runs really smooth and fine without high cpu 
> usage.
>
> So after I finished my project and moved this VM to our cluster in our 
> company, the the vm is running big slowly and has a high cpu usage.
> The servers there uses 2  CPU E5-2683 v4.

> So my big question here is, is there a supported list of cpus where I could 
> determinate if an virtual aarch64 vm will run smoothly ?

> Or a list of cpu feature flags when we would like to emulate aarch64?

AArch64 guest on x86-64 host will always be emulated. There
is not going to be a huge performance difference between
one type of x86 host CPU and another, beyond the usual
"if the CPU is faster then the emulation will run faster".

Googling suggests that the E5-2683 is a bit less than half the speed of
the AMD (unsurprising, it's a 5 year old core vs a brand new one).
If you're seeing about a 2x to 2.5x speed difference, that's
about what you should expect.

If you're seeing more than that, then probably something else is the
cause. If so, I would start by checking that everything else in the
environment is the same -- eg that it is the same QEMU version on both
machines, that in both cases there is enough memory that the system
is not running into swap, etc.

If it is just the 2x slowdown that you will naturally get on a
2x slower host CPU, then if you're currently running an older QEMU
version then you might try benchmarking the same workload with the
latest QEMU release -- we have done some optimization of codegen
over the past year or two. Don't expect miracles, but it might
make things a little better.

-- PMM