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

[Andrew Jones]

On Mon, Mar 22, 2021 at 04:42:23PM +0100, Andrea Bolognani wrote:
> 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?

I 100% agree. We should control this feature with a CPU feature property.
NV is a CPU feature, after all. Also, we should add it to the properties
that we can probe in cpu_model_advertised_features[].

Thanks,
drew

> 
> 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
> 
>