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

[Haibo Xu]

On Tue, 23 Mar 2021 at 00:32, Andrew Jones <drjones@redhat.com> wrote:
>
> 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 so much for the comments!

Yes, NV should be a vCPU feature, just as the kernel macro
KVM_ARM_VCPU_HAS_EL2 implies.
To implement it as a VM feature here just want to reuse the codes in
TCG mode which emulate a
guest CPU with virtualization extension support.

I will change the NV in KVM mode to a vCPU feature in  the next version.
@Peter Maydell and @Richard Henderson, shall we change that in TCG mode as well?

Thanks,
Haibo

> 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
> >
> >
>