Subject: ARM: KVM/XEN: how should we support virt-what?

From: [email-masked] (Marc Zyngier)
Date: Wed, 25 Mar 2015 12:03:58 +0000

On 25/03/15 09:44, Andrew Jones wrote:
> Hello ARM virt maintainers,
> 
> I'd like to start a discussion about supporting virt-what[1]. virt-what
> allows userspace to determine if the system it's running on is running
> in a guest, and of what type (KVM, Xen, etc.). Despite it being a best
> effort tool, see the Caveat emptor in [1], it has become quite a useful
> tool, and is showing up in different places, such as OpenStack. If you
> look at the code[2], specifically [3], then you'll see how it works on
> x86, which is to use the dedicated hypervisor cpuid leaves. I'm
> wondering what equivalent we have, or can develop, for arm.
> Here are some thoughts;
> 0) there's already something we can use, and I just need to be told
>    about it.
> 1) be as similar as possible to x86 by dedicating some currently
>    undefined sysreg bits. This would take buy-in from lots of parties,
>    so is not likely the way to go.
> 2) create a specific DT node that will get exposed through sysfs, or
>    somewhere.
> 3) same as (2), but just use the nodes currently in mach-virt's DT
>    as the indication we're a guest. This would just be a heuristic,
>    i.e. "have virtio mmio" && psci.method == hvc, or something,
>    and we'd still need a way to know if we're kvm vs. xen vs. ??.

KVM doesn't have any specific DT node so far (at least kvmtool doesn't
expose anything, and QEMU doesn't seem to do it either).

You could define KVM as being !Xen by looking for the Xen hypervisor
node, but that's quite a heuristic.

Overall, I'm quite reluctant to introduce things that would immediately
become an ABI (cue the recent bogomips flame wars), forming a contract
between the host userspace and the guest userspace.

	M.
-- 
Jazz is not dead. It just smells funny...