Re: [PATCH 0/3] target/arm: Add nested virtualization support

[Andrea Bolognani <abologna@redhat.com>]

On Mon, 2021-03-22 at 10:07 +0000, Haibo Xu wrote:
> This series add support for ARMv8.3/8.4 nested virtualization support
> in KVM mode. It's based on Marc Zyngier's kernel KVM patches[1], and
> has been tested on a FVP model to run a L2 guest with Qemu. Now the
> feature can be enabled by "-M virt,accel=kvm,virtualization=on" when
> starting a VM.

Why the need to enable this explicitly? AFAIK, that's not necessary
for any other architecture: on x86, you just need to make sure you're
using '-cpu host' and pass a parameter to the kernel module.

Even assuming this can't be enabled transparently, wouldn't its
availability it be controlled by a CPU feature flag, similar to what
already happens for SVE and PMU, rather than a machine type option?

That would also address the discoverability issue: unless I'm
mistaken (which I very well might be :), with the current
implementation there's no way to tell whether nested KVM will be
usable short of trying and seeing whether QEMU errors out.

-- 
Andrea Bolognani / Red Hat / Virtualization