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

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On 21/06/2019 10:37, Marc Zyngier wrote:
> From: Jintack Lim <jintack.lim@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
> Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxx>
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
>  .../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



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