Re: [PATCH 03/59] arm64: Add ARM64_HAS_NESTED_VIRT cpufeature

Julien Thierry
2019-06-21T13:08:19Z

On 21/06/2019 10:37, Marc Zyngier wrote:
> From: Jintack Lim <jintack.lim@linaro.org>
>
> Add a new ARM64_HAS_NESTED_VIRT feature to indicate that the
> CPU has the ARMv8.3 nested virtualization capability.
>
> This will be used to support nested virtualization in KVM.
>
> Signed-off-by: Jintack Lim <jintack.lim@linaro.org>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> .../admin-guide/kernel-parameters.txt | 4 +++
> arch/arm64/include/asm/cpucaps.h | 3 ++-
> arch/arm64/include/asm/sysreg.h | 1 +
> arch/arm64/kernel/cpufeature.c | 26 +++++++++++++++++++
> 4 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 138f6664b2e2..202bb2115d83 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2046,6 +2046,10 @@
> [KVM,ARM] Allow use of GICv4 for direct injection of
> LPIs.
>
> + kvm-arm.nested=
> + [KVM,ARM] Allow nested virtualization in KVM/ARM.
> + Default is 0 (disabled)
> +

Once the kernel has been built with nested guest support, what do we
gain from having it disabled by default?

It seems a bit odd since the guests have to opt-in for the capability of
running guests of their own.

Is it it likely to have negative impact a negative impact on the host
kernel? Or on guests that do not request use of nested virt?

If not I feel that this kernel parameter should be dropped.

Cheers,

--
Julien Thierry