Re: [RFC v2 0/5] ARM Nested Virt Support

[Peter Maydell]

On Fri, 9 Feb 2024 at 16:00, Eric Auger <eric.auger@redhat.com> wrote:
>
> This series adds ARM Nested Virtualization support in KVM mode.
> This is a respin of previous contributions from Miguel [1] and Haibo [2].
>
> This was tested with Marc's v11 [3] on Ampere HW with fedora L1 guest and
> L2 guests booted without EDK2. However it does not work yet with
> EDK2 but it looks unrelated to this qemu integration (host hard lockups).
>
> The host needs to be booted with "kvm-arm.mode=nested" option and
> qemu needs to be invoked with :
>
> -machine virt,virtualization=on
>
> There is a known issue with hosts supporting SVE. Kernel does not support both
> SVE and NV2 and the current qemu integration has an issue with the
> scratch_host_vcpu startup because both are enabled if exposed by the kernel.
> This is independent on whether sve is disabled on the command line. 
> Unfortunately
> I lost access to the HW that expose that issue so I couldn't fix it in this
> version.

You can probably repro that by running the whole setup under
QEMU's FEAT_NV emulation, which will be able to give you a CPU
with both FEAT_NV and SVE.

Personally I think that this is a kernel missing-feature that
should really be fixed as part of getting the kernel patches
upstreamed. There's no cause to force every userspace VMM to
develop extra complications for this.

thanks
-- PMM