Re: [PATCH v5 5/5] hw/arm/virt: Allow virt extensions with KVM

[Alyssa Ross <hi@alyssa.is>]

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Eric Auger <eric.auger@redhat.com> writes:

> From: Haibo Xu <haibo.xu@linaro.org>
>
> Up to now virt support on guest has been only supported with TCG.
> Now it becomes feasible to use it with KVM acceleration.
>
> Also check only in-kernel GICv3 is used along with KVM EL2.
>
> Signed-off-by: Haibo Xu <haibo.xu@linaro.org>
> Signed-off-by: Miguel Luis <miguel.luis@oracle.com>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Hi!  From what I can tell, this will produce an error on hosts that
don't support nested virtualization when QEMU is invoked with -accel
kvm:tcg -machine virtualization=on, but I don't think that's the ideal
behaviour.  It would make more sense for it to fall back to the first
permitted accel option that does support running the machine as
configured, so if hardware nested virtualization is not supported, it
should fall back to TCG.

I maintain an OS development environment that includes scripts for
running images in QEMU, where running KVM on those images is a
requirement.  Currently, those scripts simply force TCG on aarch64.
With this change, to take advantage of KVM NV support, I'd have to try
to identify in the script whether NV would be supported.  QEMU would be
in a much better position to determine this and fall back to TCG if it's
unsupported, like how the -accel option with multiple values usually
works.

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